Report #568

DIGHEMIV SURVEY

FOR

BATTLE MOUNTAIN EXPLORATION COMPANY

UNGA & POPOF ISLANDS

ALASKA, U.S.A.

DIGHEM SURVEYS & PROCESSING INC.	David E. Pritchard

MISSISSAUGA, ONTARIO	Geophysicist

September 21, 1990

A0568SEP.91R

SUMMARY

This report describes the logistics and results of a DIGHEMIV airborne geophysical survey carried out for Battle Mountain Exploration Company over two properties located on Unga Island and Popof Island, Alaska, U.S.A. Total coverage of the survey blocks amounted to 512 miles. The survey was flown from June 20 to July 6, 1990.

The purpose of the survey was to detect zones of conductive mineralization and to provide information that could be used to map the geology and structure of the survey areas. This was accomplished by using a DIGHEMIV multicoil, multi-frequency electromagnetic system, supplemented by a high sensitivity Cesium magnetometer and a two-channel VLF receiver. The information from these sensors was processed to produce maps which display the magnetic and conductive properties of the survey areas. An electronic navigation system, operating in the UHF band, ensured accurate positioning of the geophysical data with respect to the base maps. Visual flight path recovery techniques were used in areas where transponder signals were blocked by topographic features.

The survey properties contain many anomalous features, some of which are considered to be of moderate to high priority as exploration targets. Most of the inferred bedrock conductors appear to warrant further investigation using appropriate surface

exploration techniques. Areas of interest may be assigned priorities on the basis of supporting geophysical, geochemical and/or geological information. After initial investigations have been carried out, it may be necessary to re-evaluate the remaining anomalies based on information acquired from the follow-up program.

CONTENTS

Page

1.0 INTRODUCTION....................................... 1

2.0 SURVEY EQUIPMENT................................... 3

3.0 PRODUCTS AND PROCESSING TECHNIQUES................. 10

4.0 SURVEY RESULTS..................................... 15

GENERAL DISCUSSION

CONDUCTORS IN THE SURVEY AREAS

Unga Block

Popof Block

5.0 BACKGROUND INFORMATION............................. 27

ELECTROMAGNETICS

MAGNETICS

VLF

6.0 CONCLUSIONS AND RECOMMENDATIONS.................... 45

APPENDICES.............................................	47

A. List of Personnel

B. Statement of Cost (not included here)

C. EM Anomaly List (included as ascii file see Readme)

1.0 INTRODUCTION

A DIGHEMIV electromagnetic/resistivity/magnetic/VLF survey

was flown for Battle Mountain Exploration Company from June 20 to

July 6, 1990, over two survey blocks located on Unga Island and

Popof Island, Alaska, U.S.A. The survey areas can be located on

Port Moller Alaska map sheets Al-4 and B1-4 (Figure 1).

Survey coverage consisted of approximately 512 line- miles,

including tie lines. Flight lines were flown in azimuthal

directions of 135/315 degrees for the Unga block, and 090/270 degrees for the Popof block, with line separations of 500 ft and 600 ft, respectively.

The survey employed the DIGHEMIV electromagnetic system.

Ancillary equipment consisted of a magnetometer, radar altimeter,

video camera, analog and digital recorders, a VLF receiver and an

electronic navigation system.

The instrumentation was installed in an Aerospatiale AS350B

turbine helicopter (Registration N57955) which was provided by

Temsco Helicopters Ltd. The helicopter flew at an average

airspeed of 43 mph with an EM bird height of approximately 100

ft. Noise levels of less than 2 ppm are generally maintained for

wind speeds up to 25 mph. Higher winds may cause the system to be

grounded because excessive bird swinging produces difficulties in

flying the helicopter. The swinging results from the 50 sq. ft.

of area which is presented by the bird to broadside gusts.

In some portions of the survey blocks, the steep topography

forced the pilot to exceed normal terrain clearance for reasons

of safety. It is possible that some weak conductors may have

escaped detection in areas where the bird height exceeded 300 ft.

In difficult areas where near vertical climbs were necessary, the

forward speed of the helicopter was reduced to a level which

permitted excessive bird swinging. This problem, combined with

the severe stresses to which the bird was subjected, gave rise to

aerodynamic noise levels which are slightly higher than normal.

Where warranted, re-flights were carried out to minimize these

adverse effects.

2.0 SURVEY EQUIPMENT

This section provides a brief description of the geophysical instruments used to acquire the survey data:

Electromagnetic System

Model: DIGHEMIV

Type:	Towed bird, symmetric dipole configuration operated at a nominal survey altitude of 100 feet. Coil separation is 26 feet for 900 Hz and 7200 Hz, and 21 feet for the 56,000 Hz coil-pair.

Coil orientations/ nominal frequencies:

coaxial / 900 Hz

coplanar/ 900 Hz

coplanar/ 7,200 Hz

coplanar/56,000 Hz

Channels recorded:

4 inphase channels

4 quadrature channels

2 monitor channels

Sensitivity:

0.2 ppm at 900 Hz

0.4 ppm at 7,200 Hz

1.0 ppm at 56,000 Hz

Sample rate: 10 per second

The electromagnetic system utilizes a multi-coil coaxial/coplanar technique to energize conductors in different directions. The coaxial transmitter coil is vertical with its axis in the flight direction. The coplanar coils are horizontal. The secondary fields are sensed simultaneously by means of receiver coils which are maximum coupled to their respective transmitter coils. The system yields an inphase and a quadrature channel from each transmitter-receiver coil-pair.

Magnetometer

Model: Picodas 3340

Type: Optically pumped Cesium vapour

Sensitivity: 0.01 nT

Sample rate: 10 per second

The magnetometer sensor is towed in a bird 50 feet below the helicopter.

Magnetic Base Station

Model: Scintrex MP-3

Type: Digital recording proton precession

Sensitivity: 0.10 nT

Sample rate: 0.2 per second

A digital recorder is operated in conjunction with the base station magnetometer to record the diurnal variations of the earth's magnetic field. The clock of the base station is synchronized with that of the airborne system to permit subsequent removal of diurnal drift.

VLF System

Manufacturer: Herz Industries Ltd.

Type: Totem-2A

Sensitivity: 0.1%

Stations: Yosami, Japan NDT, 17.4 kHz

Lualualei, Hawaii NPM, 23.4 kHz

The VLF receiver measures the total field and vertical quadrature components of the secondary VLF field. Signals from two separate transmitters can be measured simultaneously. The VLF sensor is towed in a bird 30 feet below the helicopter.

Radar Altimeter

Manufacturer: Honeywell/Sperry

Type: AA 220

Sensitivity: 1 ft

The radar altimeter measures the vertical distance between the helicopter and the ground. This information is used in the processing algorithm which determines conductor depth.

Analog Recorder

Manufacturer: RMS Instruments

Type: DGR33 dot-matrix graphics recorder

Resolution: 100 x 100 dots/inch

Speed: 0.06 in/sec.

The analog profiles were recorded on chart paper in the aircraft during the survey. Table 2-1 lists the geophysical data channels and the vertical (metric) scale of each profile.

Table 2-1 The Analog profiles

Channel Name	Parameter	Scale

units/mm	Designation on

digital profile
1X9I	coaxial inphase (900 Hz)	2.5 ppm	CXI (900Hz)
1X9Q	coaxial quadrature (900 Hz)	2.5 ppm	CXQ (900Hz)
3P9I	coplaner inphase (900 Hz)	2.5 ppm	CPI (900Hz)
3P9Q	coplaner quadrature (900 Hz)	2.5 ppm	CPQ (900Hz)
2P7I	coplaner inphase	5 ppm	CPI (7200Hz)
2P7Q	coplaner quadrature	5 ppm	CPQ (7200Hz)
4P5I	coplaner inphase	10 ppm	CPI (56000Hz)
4P5Q	coplaner quadrature	10 ppm	CPQ (56000Hz)
ALTR	altimeter	3 m	ALT
CMGC	magnetics, coarse	25 nt	MAG
CMGF	magnetics, fine	2.5 nt
VF1T	VLF - total: primary station	2%
VF1Q	VLF - quad: primary station	2%
VF2T	VLF - total: secondary station	2%
VF2Q	VLF - quad: secondary station	2%
CXSP	coaxial spherics monitor	CXS
CPSP	coplanar spherics monitor	CPS
CXPL	coaxial powerline monitor	CXP
CPPL	coplanar powerline monitor	CPP

Digital Data Acquisition System

Manufacturer: RMS

Instruments Type: DGR 33

Tape Deck: RMS TCR-12, 6400 bpi, tape cartridge recorder

The digital data were used to generate several computed parameters. Both measured and computed parameters were plotted as "multi-channel stacked profiles" during data processing. These parameters are shown in Table 2-2.

In Table 2-2 the log resistivity scale of 0.06 decade/mm means that the resistivity changes by an order of magnitude in 16.6 mm. The resistivities at 0, 33 and 67 mm up from the bottom of the digital profile are respectively 1, 100 and 10,000 ohm-m.

Tracking Camera

Type: Panasonic Video Model: AG 2400/WVCD132

Fiducial numbers are recorded continuously and are displayed on the margin of each image. This procedure ensures accurate correlation of analog and digital data with respect to visible features on the ground.

Navigation System

Model:	Del Norte 547

Type:	UHF electronic positioning system

Sensitivity:	3 ft.

Sample rate:	2 per second

The navigation system uses ground based transponder stations which transmit distance information back to the helicopter. The ground stations are set up well away from the survey area and are positioned such that the signals cross the survey block at an angle between 30 and 150 degrees. The onboard Central Processing Unit takes any two transponder distances and determines the

Table 2-2 The Digital Profiles

Channel Name (Frequency)

Observed Parameters	Scale

units/mm
MAG	magnetics	10 nT
ALT	bird height	6 m
CXI (900 Hz)	vertical coaxial coil-pair inphase	2 ppm
CXQ (900 Hz)	vertical coaxial coil-pair quadrature	2 ppm
CPI (900 Hz)	horizontal coplanar coil-pair inphase	2 ppm
CPQ (900 Hz)	horizontal coplanar coil-pair quadrature	2 ppm
CPI (7200 Hz)	horizontal coplanar coil-pair inphase	4 ppm
CPQ (7200 Hz)	horizontal coplanar coil-pair quadrature	4 ppm
CPI (56 kHz)	horizontal coplanar coil-pair inphase	10 ppm
CPQ (56 kHz)	horizontal coplanar coil-pair quadrature	10 ppm
CXS	coaxial spherics monitor
CXP	coaxial powerline monitor
CPS	coplanar spherics monitor
CPP	coplaner powerline monitor
Computed Parameters
DFI (900Hz)	difference function inphase from CXI and CPI	2 ppm
DFQ (900Hz)	difference function quadrature from CXI and CPI	2 ppm
RES (900Hz)	log resistivity	.06 decade
RES (7200 Hz)	log resistivity	.06 decade
RES (56 kHz)	log resistivity	.06 decade
DP (900Hz)	apparent depth	6 m
DP (7200Hz)	apparent depth	6 m
DP (56 kHz)	apparent depth	6 m
CDT	conductance (see text)	1 grade

helicopter position relative to these two ground stations in cartesian coordinates.	The cartesian coordinates are transformed to UTM coordinates during data processing. This is accomplished by correlating a number of prominent topographical locations with the navigational data points. The use of numerous visual tie points serves two purposes: to accurately relate the navigation data to the map sheet and to minimize location errors which might result from distortions in uncontrolled photo-mosaic base maps.

3.0 PRODUCTS AND PROCESSING TECHNIQUES

The following products are available from the survey data. Those which are not part of the survey contract may be acquired later. Refer to Table 3-1 for a summary of the maps which accompany this report, some of which may be sent under separate cover (editor - only select maps provided here; other maps may be available from the Aleut Corporation). Most parameters can be displayed as contours, profiles, or in color.

Table 3-1 Plots Available from Survey

MAP PRODUCT	NO. OF SHEETS	ANOMALY MAP	PROFILES ON MAP	CONTOURS	SHADOW MAP
INK	COLOR
Electromagnetic anomalies	2	12,000	N/A	N/A	N/A	N/A
Probable Bedrock conductors	-	N/A	N/A	N/A	N/A	N/A
Resistivity (900 Hz)1	-	N/A	-	-	-	-
Resistivity (7,200 Hz)	2	N/A	-	12,000	12,000	-
Resistivity (56,000 Hz)1	-	N/A	-	-	-	-
EM Magnetite	-	N/A	-	-	-	-
Total Field Magnetics	2	N/A	-	12,000	12,000	-
Enhanced Magnetics	-	N/A	-	-	-	-
1st Vertical Derivative Magnetics	2	N/A	-	12,000	12,000	-
2nd Vertical Derivative Magnetics	-	N/A	-	-	-	-
Filtered Total Field VLF	2	N/A	-	12,000	12,000	-
VLF Profiles	-	N/A	-	-	-	-
Electromagnetic Profiles (900 Hz)	-	N/A	-	-	-
Electromagnetic Profiles

(7200 Hz)	-	N/A	-	-	-	-

N/A	Not Available this contract

Not required this contract

12,000	scale of delivered map

1

resistivities computed by contractor as part of new USGS work

NOTES:

Inked contour maps are provided on transparent media and show flight lines, EM anomalies, and suitable registration. Two paper prints of each map are supplied. (Editor - these were with the original contract)

Base Maps

Base maps of the survey areas have been produced from published topographic maps. These provide a relatively accurate, distortion-free base which facilitates correlation of the navigation data to the UTM grid. Photomosaics are useful for visual reference and for subsequent flight path recovery, but usually contain scale distortions. Orthophotos are ideal, but their cost and the time required to produce them, usually precludes their use as base maps.

Electromagnetic Anomalies

Anomalous electromagnetic responses are selected and analyzed by computer to provide a preliminary electromagnetic anomaly map. This preliminary map is used, by the geophysicist, in conjunction with the computer-generated digital profiles, to produce the final interpreted EM anomaly map. This map includes bedrock, surficial and cultural conductors. A map containing only bedrock conductors can be generated, if desired.

Resistivity

The apparent resistivity in ohm-m may be generated from the inphase and quadrature EM components for any of the frequencies, using a pseudo-layer half-space model. A resistivity map portrays all the EM information for that frequency over the entire survey area. This contrasts with the electromagnetic anomaly map which provides information only over interpreted conductors. The large dynamic range makes the resistivity parameter an excellent mapping tool.

EM Magnetite

The apparent percent magnetite by weight is computed wherever magnetite produces a negative inphase EM response. This parameter was not computed for this survey.

Total Field Magnetics

The aeromagnetic data are corrected for diurnal variation using the magnetic base station data. The regional IGRF can be removed from the data, if requested.

Enhanced Magnetics

The total field magnetic data are subjected to a processing algorithm. This algorithm enhances the response of magnetic bodies in the upper 1500 ft and attenuates the response of deeper bodies. The resulting enhanced magnetic map provides better definition and resolution of near surface magnetic units. It also identifies weak magnetic features which may not be evident on the total field magnetic map. However, regional magnetic variations, and magnetic lows caused by remanence, are better defined on the total field magnetic map. The technique is described in more detail in the following section on background information.

Magnetic Derivatives

The total field magnetic data may be subjected to a variety of filtering techniques to yield maps of the following:

vertical gradient

second vertical derivative

magnetic susceptibility with reduction to the pole upward/downward continuations

All of these filtering techniques improve the recognition of near-surface magnetic bodies, with the exception of upward continuation. Any of these parameters can be produced on request. Dighem's proprietary enhanced magnetic technique is designed to provide a general "all-purpose" map, combining the more useful features of the above parameters.

VLF

The VLF data are digitally filtered to remove long wavelengths such as those caused by variations in the transmitted field strength.

Multi-channel Stacked Profiles (not included in this release)

Distance-based profiles of the digitally recorded geophysical data are generated and plotted by computer. These profiles also contain the calculated parameters which are used in the interpretation process. These are produced as worksheets prior to interpretation, and can also be presented in the final corrected form after interpretation. The profiles display electromagnetic anomalies with their respective interpretive symbols. The differences between the worksheets and the final corrected form occur only with respect to the EM anomaly identifier.

Contour, Color and Shadow Map Displays

The geophysical data are interpolated onto a regular grid using an Akima spline technique. The resulting grid is suitable for generating contour maps of excellent quality.

Color maps are produced by interpolating the grid down to the pixel size. The parameter is then incremented with respect to specific amplitude ranges to provide color "contour" maps. Color maps of the total magnetic field are particularly useful in defining the lithology of the survey area.

Monochromatic shadow maps are generated by employing an artificial sun to cast shadows on a surface defined by the geophysical grid. There are many variations in the shadowing technique. These techniques may be applied to total field or enhanced magnetic data, magnetic derivatives, VLF, resistivity, etc. Of the various magnetic products, the shadow of the enhanced magnetic parameter is particularly suited for defining geological structures with crisper images and improved resolution.

4.0 SURVEY RESULTS

GENERAL DISCUSSION

The survey results are presented on 2 separate map sheets for each parameter at a scale of 1:12,000 (the map sheets are in accompanying bit-map files). Tables 4-1 and 4-2 summarize the EM responses in the survey areas, with respect to conductance grade and interpretation.

The anomalies shown on the electromagnetic anomaly maps are based on a near-vertical, half plane model. This model best reflects "discrete" bedrock conductors. Wide bedrock conductors or flat-lying conductive units, whether from surficial or bedrock sources, may give rise to very broad anomalous responses on the EM profiles. These may not appear on the electromagnetic anomaly map if they have a regional character rather than a locally anomalous character. These broad conductors, which more closely approximate a half space model, will be maximum coupled to the horizontal (coplanar) coil-pair and should be more evident on the resistivity parameter. Resistivity maps, therefore, may be more valuable than the electromagnetic anomaly maps, in areas where broad or flat-lying conductors are considered to be of importance. Contoured resistivity maps, based on the 7200 Hz coplanar data, are included with this report.

TABLE 4-1

EM ANOMALY STATISTICS UNGA BLOCK
CONDUCTOR

GRADE	CONDUCTANCE RANGE

SIEMENS (MHOS)	NUMBER OF

RESPONSES
7	>100	1
6	50-100	2
5	20-50	19
4	10-20	80
3	5-10	90
2	1-5	369
1	<1	130
*	INDETERMINATE	231
TOTAL	1022
CONDUCTOR TYPE

(FIGURE 2)	MOST LIKELY SOURCE

MODEL	NUMBER OF

RESPONSES
D	NARROW BED ROCK CONDUCTOR	6
B	BEDROCK CONDUCTOR	729
S	CONDUCTIVE COVER	276
H	ROCK UNIT OR THICK COVER	10
E	EDGE OF CONDUCTOR	-
TOTAL	1022

TABLE 4-2

EM ANOMALY STATISTICS POPOV BLOCK
CONDUCTOR

GRADE	CONDUCTANCE RANGE

SIEMENS (MHOS)	NUMBER OF

RESPONSES
7	>100	0
6	50-100	1
5	20-50	4
4	10-20	31
3	5-10	60
2	1-5	101
1	<1	36
*	INDETERMINATE	78
TOTAL	311
CONDUCTOR TYPE

(FIGURE 2)	MOST LIKELY SOURCE

MODEL	NUMBER OF

RESPONSES
D	NARROW BED ROCK CONDUCTOR	-
B	BEDROCK CONDUCTOR	202
S	CONDUCTIVE COVER	109
H	ROCK UNIT OR THICK COVER	-
E	EDGE OF CONDUCTOR	-
TOTAL	311

Further explanations on EM anomaly map; image popvm.tif

Excellent resolution and discrimination of conductors was accomplished by using a fast sampling rate of 0.1 sec and by employing a common frequency (900 Hz) on two orthogonal coil- pairs (coaxial and coplanar). The resulting "difference channel" parameters often permit differentiation of bedrock and surficial conductors, even though they may exhibit similar conductance values.

Anomalies which occur near the ends of the survey lines (i.e., outside the survey areas), should be viewed with caution. Some of the weaker anomalies could be due to aerodynamic noise, i.e., bird bending, which is created by abnormal stresses to which the bird is subjected during the climb and turn of the aircraft between lines. Such aerodynamic noise is usually manifested by an anomaly on the coaxial inphase channel only, although severe stresses can affect the coplanar inphase channels as well.

Magnetics

A Scintrex MP-3 proton precession magnetometer was operated at

the survey base to record diurnal variations of the earth's

magnetic field. The clock of the base station was synchronized

with that of the airborne system to permit subsequent removal of

diurnal drift.

The background magnetic level has been adjusted to match the International Geomagnetic Reference Field (IGRF) for the survey area. The IGRF gradient across each survey block is left intact. This procedure ensures that the magnetic contours will match contours from any adjacent surveys which have been processed in a similar manner.

The total field magnetic data have been presented as contours on the base maps (not included with this data release) using a contour interval of 10 nT where gradients permit. The maps show the magnetic properties of the rock units underlying the survey areas.

The total field magnetic data have been subjected to a processing algorithm to produce calculated vertical gradient magnetic maps. This procedure enhances near-surface magnetic units and suppresses regional gradients. It also provides better definition and resolution of magnetic units and displays weak magnetic features which may not be clearly evident on the total field maps.

There is some evidence on the magnetic maps which suggests that the survey areas have been subjected to deformation and/or alteration. These structural complexities are evident on the contour maps as variations in magnetic intensity, irregular patterns, and as offsets or changes in strike direction. Some of the more prominent linear features are evident as topographic features and are also displayed by VLF conductors.

If a specific magnetic intensity can be assigned to the rock type which is believed to host the target mineralization, it may be possible to select areas of higher priority on the basis of the total field magnetic data. This is based on the assumption that the magnetite content of the host rocks will give rise to a limited range of contour values which will permit differentiation of various lithological units.

The magnetic results, in conjunction with the other geophysical parameters, should provide valuable information which can be used to effectively map the geology and structure in the survey areas.

VLF

VLF results were obtained from the transmitting stations at Yosami, Japan (NDT - 17.4 kHz), and Lualualei, Hawaii (NPM - 23.4 kHz). The VLF maps show the contoured results of the filtered total field from NDT for the Unga Island area, and NPM for the Popof Island area.

The VLF method is quite sensitive to the angle of coupling between the conductor and the propagated EM field. Consequently, conductors which strike towards the VLF station will usually yield a stronger response than conductors which are nearly orthogonal to it. The general northeast/southwest strike in Unga, and the north/south strike in Popof, provide moderately good coupling with the VLF fields from Japan and Hawaii, respectively.

The VLF parameter does not normally provide the same degree of resolution available from the EM data. Closely spaced conductors, conductors of short strike length or conductors which are poorly coupled to the VLF field, may escape detection with this method. Erratic signals from the VLF transmitters can also give rise to strong, isolated anomalies which should be viewed with caution. Regardless of these limitations, however, the VLF results have provided some additional information, primarily within the more resistive portions of the survey areas. The VLF method could probably be used as a follow-up tool in some areas, although its effectiveness will be severely limited in areas of moderate to high conductivity. The filtered total field VLF contours are presented on the base maps (not supplied with this data release) with a contour interval of one percent.

Resistivity

Resistivity maps, which display the conductive properties of the survey area, were produced from the 7200 Hz coplanar data. In general, the resistivity patterns show moderately poor agreement with the magnetic trends. This suggests that many of the resistivity lows are probably related to surficial features, rather than bedrock units. Note, for example, the close correlation between the 10 ohm-m resistivity contour and the coastal shoreline on the Popof grid. There are a few areas, however, where contour patterns appear to be due to bedrock conductors. Some of these are quite extensive and often reflect "formational" conductors which may be of minor interest as direct exploration targets. However, attention may be focused on areas where these zones appear to be faulted or folded or where anomaly characteristics differ along strike.

Electromagnetics

The EM anomalies resulting from this survey appear to fall into two general categories. The first type consists of discrete, well-defined anomalies which yield marked inflections on the difference channels. These anomalies are usually attributed to conductive sulphides or graphite and are generally given a "B", "T" or "D" interpretive symbol, denoting a bedrock source.

The second class of anomalies comprises moderately broad responses which exhibit the characteristics of a half space and do not yield well-defined inflections on the difference channels. Anomalies in this category are usually given an "S" or "H" interpretive symbol. The lack of a difference channel response usually implies a broad or flat-lying conductive source such as overburden. Some of these anomalies may reflect conductive rock units or zones of deep weathering, but most are attributed to salt water.

The effects of conductive overburden are evident over portions of the survey areas. Although the difference channels (DFI and DFQ) are extremely valuable in detecting bedrock conductors which are partially masked by conductive overburden, sharp undulations in the bedrock/overburden interface can yield anomalies in the difference channels which may be interpreted as possible bedrock conductors. Such anomalies usually fall into the "S?" or "B?" classification but may also be given an "E" interpretive symbol, denoting a resistivity contrast at the edge of a conductive unit.

In areas where EM responses are evident primarily on the quadrature components, zones of poor conductivity are indicated. Where these responses are coincident with magnetic anomalies, it is possible that the inphase component amplitudes have been suppressed by the effects of magnetite. Most of these poorly-conductive magnetic features give rise to resistivity anomalies which are only slightly below background. If it is expected that poorly-conductive economic mineralization may be associated with magnetite-rich units, most of these weakly anomalous features will be of interest. In areas where magnetite causes the inphase components to become negative, the apparent conductance and depth of EM anomalies may be unreliable.

As economic mineralization within the survey blocks may be associated with massive to weakly disseminated sulphides, which may or may not be hosted by magnetite-rich rocks, it is difficult to assess the relative merits of EM anomalies on the basis of conductance. It is recommended that an attempt be made to compile a suite of geophysical "signatures" over areas of interest. Anomaly characteristics are clearly defined on the computer-processed geophysical data profiles which are supplied as one of the survey products (not included in this data release).

A complete assessment and evaluation of the survey data should be carried out by one or more qualified professionals who have access to, and can provide a meaningful compilation of, all available geophysical, geological and geochemical data.

CONDUCTORS IN THE SURVEY AREAS

The electromagnetic anomaly maps show the anomaly locations with the interpreted conductor type, dip, conductance and depth being indicated by symbols. Direct magnetic correlation is also shown if it exists. The strike direction and length of the conductors are indicated when anomalies can be correlated from line to line. When studying the map sheets, consult the anomaly listings appended to this report (included as an ascii file - see digital data directory).

In areas where several conductors or conductive trends appear to be related to a common geological unit, these have been outlined as "zones" on the EM anomaly maps. The zone outlines usually approximate the limits of conductive units defined by the resistivity contours, but may also be related to distinct rock units which may be inferred from the magnetic data.

UNGA BLOCK

Many of the magnetically active zones in the Unga block correlate with resistive zones on the 7200 Hz resistivity map. This is usually due to the effects of magnetite. In these areas weak conductors may yield quadrature responses only. Other features display a conductive nature and are associated with well-defined resistivity lows.

The general strike of EM conductors in the Unga survey block trend approximately northeast/southwest. These conductors have line to line correlation and exhibit strike lengths from 500 ft to one mile. Several conductors show as single line responses. There are two zones of major interest which have been outlined on the electromagnetic anomaly map.

Zone 1

The central section of the grid contains a cluster of conductors trending northeast/ southwest between lines 10010 and 10650. This zone is not apparent on the resistivity map, but appears as a very active magnetic unit on the total field magnetic map. Nearly all of these conductors are attributed to bedrock sources. Magnetic correlation is variable, indicating a change in the conductor source along strike.

Zone 2

Zone 2 lies to the south and east and contains conductors striking east/west. These tend to cross-cut the general geologic strike. They do, however, follow the magnetics of the grid and appear to be associated with a magnetic break which trends west from fiducial 1193 on line 10650 to fiducial 3000 on line 10390. This may represent an area of structural alteration which may be of interest.

POPOF BLOCK
(EDITORAL NOTE: The contactor's interpreted conductor map is given as file popanom.tif)

The central portion of the Popof grid is dominated by a moderately strong and complex magnetic anomaly, the central axis of which strikes approximately 0100. This unit is flanked on the north, east and west, by a relatively nonmagnetic unit. Two other bands of more magnetic rocks are evident at the eastern and western limits of the survey lines.

Magnetic relief exceeds 1500 nT, varying from a low of less than 51,000 nT to a high of more than 52,400 nT (line 20160) on a background of approximately 51,500 nT. The very erratic nature of the magnetic contour patterns is partially due to remanence or polarity reversals. Many of the stronger magnetic lows are obviously due to remanent magnetism rather than an absence of magnetite. Note, for example, the negative inphase responses at fiducial 886 on line 20160 which coincide with a magnetic low.

Remanence is not uncommon in Tertiary volcanics. However,

negative magnetic signatures are often associated with zones of

alteration which may be favourable areas for mineral deposition.

The resistivity patterns bear very little resemblance to the magnetic patterns except in the northwestern quadrant, where a sharp resistivity contrast coincides with an arcuate magnetic low. This probable contact extends in a gentle arc, from the western end of line 20241, southeast towards fiducial 3260 on line 20110. From this point, the 100 ohm-m resistivity contour closely follows the 51,800 nT magnetic contour in a northerly direction along the western periphery of the central magnetic complex. The relatively low magnetic unit correlates with a moderately conductive zone, and both are associated with a narrow valley which opens towards the north.

Although the conductivity contrast in this area may be enhanced by residual salinity, the corresponding magnetic low indicates a probable relationship to bedrock structure. Any

anomalies along this assumed contact may therefore be of interest. However, most are quite broad and weak, and have generally been attributed to surficial material. Most of the interpreted bedrock conductors to the east, give rise to more "discrete" resistivity lows.

There are a few "plug-like" resistivity highs evident within the survey grid. Most of these can be directly attributed to zones of high magnetite content, which has caused suppression of the inphase components. Examples occur on line 20110 at fiducial 3230, line 20200 at 2530, line 20210 at 1654, line 20230 at 320, and line 20250 at 2000. Most of these give rise to quadrature anomalies which indicate the presence of weakly conductive material.

The coastline is clearly defined by a sharp resistivity contrast. Salt water normally gives rise to resistivity values of less than 1 ohm-m. This high conductivity would mask any bedrock conductors such as 20030E and F which might extend beyond the coastline.

The VLF results over the Popof grid are of dubious value. Not only were the signals very weak, but they were also affected by elevation and altitude variations. Many of the bedrock anomalies correlate with VLF highs, but in most cases this correlation appears to be random or coincidental, rather than direct. There are a few exceptions, most of which occur on the higher, more resistive ground, in the central portion of the grid. All VLF anomalies over salt water are spurious.

The EM map shows several conductors, most of which have been attributed to probable bedrock sources. The general strike appears to be roughly east-northeast, although there are a few short conductors which may strike northwest/ southeast. These latter conductors may be associated with structural breaks which transect the local geological strike.

Magnetic correlation is variable. Most conductors yield occasional correlation along strike but none appear to be magnetic along their entire strike length. Where conductors are associated with magnetite-rich zones, the apparent conductance may be understated. Anomaly 20160D is one such example.

The effects of magnetite and the presence of conductive surficial material have both tended to distort or mask the individual anomaly characteristics. Therefore, it is unreasonable to assess the relative merits of anomalies on the basis of either shape or conductance. If a specific geophysical signature can be attributed to known mineralization, it is possible that this could be used as one of the criteria for defining other zones of interest.

5.0 BACKGROUND INFORMATION

This section provides background information on parameters which are available from the survey data. Those which have not been supplied as survey products may be generated later from raw data on the digital archive tape.

ELECTROMAGNETICS

DIGHEM electromagnetic responses fall into two general classes, discrete and broad. The discrete class consists of sharp, well-defined anomalies from discrete conductors such as sulfide lenses and steeply dipping sheets of graphite and sulfides. The broad class consists of wide anomalies from conductors having a large horizontal surface such as flatly dipping graphite or sulfide sheets, saline water-saturated sedimentary formations, conductive overburden and rock, and geothermal zones. A vertical conductive slab with a width of 200 m would straddle these two classes.

The vertical sheet (half plane) is the most common model used for the analysis of discrete conductors. All anomalies plotted on the electromagnetic map are analyzed according to this model. The following section entitled Discrete Conductor Analysis describes this model in detail, including the effect of using it on anomalies caused by broad conductors such as conductive overburden.

The conductive earth (half space) model is suitable for broad conductors. Resistivity contour maps result from the use of this model. A later section entitled Resistivity Mapping describes the method further, including the effect of using it on anomalies caused by discrete conductors such as sulfide bodies.

Geometric interpretation

The geophysical interpreter attempts to determine the geometric shape and dip of the conductor. Figure 2 shows typical DIGHEM anomaly shapes which are used to guide the geometric interpretation.

Discrete conductor analysis

The EM anomalies appearing on the electromagnetic map are analyzed by computer to give the conductance (i.e., conductivity- thickness product) in Siemens (mhos) of a vertical sheet model. This is done regardless of the interpreted geometric shape of the conductor. This is not an unreasonable procedure, because the computed conductance increases as the electrical quality of the conductor increases, regardless of its true shape. DIGHEM anomalies are divided into seven grades of conductance, as shown in Table 5-1 below. The conductance in Siemens (mhos) is the reciprocal of resistance in ohms.

TABLE 5-1 EM ANOMALY GRADES

ANOMALY GRADE	SIEMENS
7	>100
6	50-100
5	20-50
4	10-20
3	5-10
2	1-5
1	<1

The conductance value is a geological parameter because it is a characteristic of the conductor alone. It generally is independent of frequency, flying height or depth of burial, apart from the averaging over a greater portion of the conductor as height increases. Small anomalies from deeply buried strong conductors are not confused with small anomalies from shallow weak conductors because the former will have larger conductance values.

Conductive overburden generally produces broad EM responses which may not be shown as anomalies on the EM maps. However, patchy conductive overburden in otherwise resistive areas can yield discrete anomalies with a conductance grade (cf. Table 5-1) of 1, 2 or even 3 for conducting clays which have resistivities as low as 50 ohm-m. In areas where ground resistivities are below 10 ohm-m, anomalies caused by weathering variations and similar causes can have any conductance grade. The anomaly shapes from the multiple coils often allow such conductors to be recognized, and these are indicated by the letters S, H, and sometimes E on the electromagnetic anomaly map (see EM map legend, image popanom.tif).

For bedrock conductors, the higher anomaly grades indicate increasingly higher conductances. Examples: DIGHEM's New Insco copper discovery (Noranda, Canada) yielded a grade 5 anomaly, as did the neighboring copper-zinc Magusi River ore body; Mattabi (copper-zinc, Sturgeon Lake, Canada) and Whistle (nickel, Sudbury, Canada) gave grade 6; and DIGHEM's Montcalm nickel- copper discovery (Timmins, Canada) yielded a grade 7 anomaly. Graphite and sulfides can span all grades but, in any particular survey area, field work may show that the different grades indicate different types of conductors.

Strong conductors (i.e., grades 6 and 7) are characteristic of massive sulfides or graphite. Moderate conductors (grades 4 and 5) typically reflect graphite or sulfides of a less massive character, while weak bedrock conductors (grades 1 to 3) can signify poorly connected graphite or heavily disseminated sulfides. Grades 1 and 2 conductors may not respond to ground EM equipment using frequencies less than 2000 Hz.

The presence of sphalerite or gangue can result in ore deposits having weak to moderate conductances. As an example, the three million ton lead-zinc deposit of Restigouche Mining Corporation near Bathurst, Canada, yielded a well-defined grade 2 conductor. The 10 percent by volume of sphalerite occurs as a coating around the fine grained massive pyrite, thereby inhibiting electrical conduction.

Faults, fractures and shear zones may produce anomalies which typically have low conductances (e.g., grades 1 to 3). Conductive rock formations can yield anomalies of any conductance grade. The conductive materials in such rock formations can be salt water, weathered products such as clays, original depositional clays, and carbonaceous material.

On the interpreted electromagnetic map, a letter identifier and an interpretive symbol are plotted beside the EM grade symbol. The horizontal rows of dots, under the interpretive symbol, indicate the anomaly amplitude on the flight record. The vertical column of dots, under the anomaly letter, gives the estimated depth. In areas where anomalies are crowded, the letter identifiers, interpretive symbols and dots may be obliterated. The EM grade symbols, however, will always be discernible, and the obliterated information can be obtained from the anomaly listing appended to this report.

The purpose of indicating the anomaly amplitude by dots is to provide an estimate of the reliability of the conductance calculation. Thus, a conductance value obtained from a large ppm anomaly (3 or 4 dots) will tend to be accurate whereas one obtained from a small ppm anomaly (no dots) could be quite inaccurate. The absence of amplitude dots indicates that the anomaly from the coaxial coil-pair is 5 ppm or less on both the inphase and quadrature channels. Such small anomalies could reflect a weak conductor at the surface or a stronger conductor at depth. The conductance grade and depth estimate illustrates which of these possibilities fits the recorded data best.

Flight line deviations occasionally yield cases where two anomalies, having similar conductance values but dramatically different depth estimates, occur close together on the same conductor. Such examples illustrate the reliability of the conductance measurement while showing that the depth estimate can be unreliable. There are a number of factors which can produce an error in the depth estimate, including the averaging of topographic variations by the altimeter, overlying conductive overburden, and the location and attitude of the conductor relative to the flight line. Conductor location and attitude can provide an erroneous depth estimate because the stronger part of the conductor may be deeper or to one side of the flight line, or because it has a shallow dip. A heavy tree cover can also produce errors in depth estimates. This is because the depth estimate is computed as the distance of bird from conductor, minus the altimeter reading. The altimeter can lock onto the top of a dense forest canopy. This situation yields an erroneously large depth estimate but does not affect the conductance estimate.

Dip symbols are used to indicate the direction of dip of conductors. These symbols are used only when the anomaly shapes are unambiguous, which usually requires a fairly resistive environment.

A further interpretation is presented on the EM map by means of the line-to-line correlation of anomalies, which is based on a comparison of anomaly shapes on adjacent lines. This provides conductor axes which may define the geological structure over portions of the survey area. The absence of conductor axes in an area implies that anomalies could not be correlated from line to line with reasonable confidence.

DIGHEM electromagnetic maps are designed to provide a correct impression of conductor quality by means of the conductance grade symbols. The symbols can stand alone with geology when planning a follow-up program. The actual conductance values are printed in the attached anomaly list for those who wish quantitative data. The anomaly ppm and depth are indicated by inconspicuous dots which should not distract from the conductor patterns, while being helpful to those who wish this information. The map provides an interpretation of conductors in terms of length, strike and dip, geometric shape, conductance, depth, and thickness. The accuracy is comparable to an interpretation from a high quality ground EM survey having the same line spacing.

The attached EM anomaly list provides a tabulation of anomalies in ppm, conductance, and depth for the vertical sheet model. The EM anomaly list also shows the conductance and depth for a thin horizontal sheet (whole plane) model, but only the vertical sheet parameters appear on the EM map. The horizontal sheet model is suitable for a flatly dipping thin bedrock conductor such as a sulfide sheet having a thickness less than 10 m. The list also shows the resistivity and depth for a conductive earth (half space) model, which is suitable for thicker slabs such as thick conductive overburden. In the EM anomaly list, a depth value of zero for the conductive earth model, in an area of thick cover, warns that the anomaly may be caused by conductive overburden.

Since discrete bodies normally are the targets of EM surveys, local base (or zero) levels are used to compute local anomaly amplitudes. This contrasts with the use of true zero levels which are used to compute true EM amplitudes. Local anomaly amplitudes are shown in the EM anomaly list and these are used to compute the vertical sheet parameters of conductance and depth. Not shown in the EM anomaly list are the true amplitudes which are used to compute the horizontal sheet and conductive earth parameters.

Questionable Anomalies

DIGHEM maps may contain EM responses which are displayed as asterisks (*). These responses denote weak anomalies of indeterminate conductance, which may reflect one of the following: a weak conductor near the surface, a strong conductor at depth (e.g., 100 to 120 m below surface) or to one side of the flight line, or aerodynamic noise. Those responses that have the appearance of valid bedrock anomalies on the flight profiles are indicated by appropriate interpretive symbols (see EM map legend). The others probably do not warrant further investigation unless their locations are of considerable geological interest.

The thickness parameter

DIGHEM can provide an indication of the thickness of a steeply dipping conductor. The amplitude of the coplanar anomaly (e.g., CPI channel on the digital profile) increases relative to the coaxial anomaly (e.g., CXI) as the apparent thickness increases, i.e., the thickness in the horizontal plane. (The thickness is equal to the conductor width if the conductor dips at 90 degrees and strikes at right angles to the flight line.) This report refers to a conductor as thin when the thickness is likely to be less than 3 m, and thick when in excess of 10 m. Thick conductors are indicated on the EM map by parentheses "( )". For base metal exploration in steeply dipping geology, thick conductors can be high priority targets because many massive sulfide ore bodies are thick, whereas non-economic bedrock conductors are often thin. The system cannot sense the thickness when the strike of the conductor is subparallel to the flight line, when the conductor has a shallow dip, when the anomaly amplitudes are small, or when the resistivity of the environment is below 100 ohm-m.

Resistivity mapping

Areas of widespread conductivity are commonly encountered during surveys. In such areas, anomalies can be generated by decreases of only 5 m in survey altitude as well as by increases in conductivity. The typical flight record in conductive areas is characterized by inphase and quadrature channels which are continuously active. Local EM peaks reflect either increases in conductivity of the earth or decreases in survey altitude. For such conductive areas, apparent resistivity profiles and contour maps are necessary for the correct interpretation of the airborne data. The advantage of the resistivity parameter is that anomalies caused by altitude changes are virtually eliminated, so the resistivity data reflect only those anomalies caused by conductivity changes. The resistivity analysis also helps the interpreter to differentiate between conductive trends in the bedrock and those patterns typical of conductive overburden. For example, discrete conductors will generally appear as narrow lows on the contour map and broad conductors (e.g., overburden) will appear as wide lows.

The resistivity profiles and the resistivity contour maps present the apparent resistivity using the so-called pseudo-layer (or buried) half space model defined by Fraser (1978)1. This model consists of a resistive layer overlying a conductive half space. The depth channels give the apparent depth below surface of the conductive material. The apparent depth is simply the apparent thickness of the overlying resistive layer. The apparent depth (or thickness) parameter will be positive when the upper layer is more resistive than the underlying material, in which case the apparent depth may be quite close to the true depth.

The apparent depth will be negative when the upper layer is more conductive than the underlying material, and will be zero when a homogeneous half space exists. The apparent depth parameter must be interpreted cautiously because it will contain any errors which may exist in the measured altitude of the EM bird (e.g., as caused by a dense tree cover). The inputs to the resistivity algorithm are the inphase and quadrature components of the coplanar coil-pair. The outputs are the apparent resistivity of the conductive half space (the source) and the sensor-source distance. The flying height is not an input variable, and the output resistivity and sensor-source distance are independent of the flying height. The apparent depth, discussed above, is simply the sensor-source distance minus the measured altitude or flying height. Consequently, errors in the measured altitude will affect the apparent depth parameter but not the apparent resistivity parameter.

The apparent depth parameter is a useful indicator of simple layering in areas lacking a heavy tree cover. The DIGHEM system has been flown for purposes of permafrost mapping, where positive apparent depths were used as a measure of permafrost thickness. However, little quantitative use has been made of negative apparent depths because the absolute value of the negative depth is not a measure of the thickness of the conductive upper layer and, therefore, is not meaningful physically. Qualitatively, a negative apparent depth estimate usually shows that the EM anomaly is caused by conductive overburden. Consequently, the apparent depth channel can be of significant help in distinguishing between overburden and bedrock conductors.

The resistivity map often yields more useful information on conductivity distributions than the EM map. In comparing the EM and resistivity maps, keep in mind the following:

(a)	The resistivity map portrays the apparent value of the earth's resistivity, where resistivity = 1/conductivity.

(b)	The EM map portrays anomalies in the earth's resistivity. An anomaly by definition is a change from the norm and so the EM map displays anomalies, (i) over narrow, conductive bodies and (ii) over the boundary zone between two wide formations of differing conductivity.

The resistivity map might be likened to a total field map and the EM map to a horizontal gradient in the direction of flight2. Because gradient maps are usually more sensitive than total field maps, the EM map therefore is to be preferred in resistive areas. However, in conductive areas, the absolute character of the resistivity map usually causes it to be more useful than the EM map.

Interpretation in conductive environments

Environments having background resistivities below 30 ohm-m cause all airborne EM systems to yield very large responses from the conductive ground. This usually prohibits the recognition of discrete bedrock conductors. However, DIGHEM data processing techniques produce three parameters which contribute significantly to the recognition of bedrock conductors. These are the inphase and quadrature difference channels (DFI and DFQ), and the resistivity and depth channels (RES and DP) for each coplanar frequency.

The EM difference channels (DFI and DFQ) eliminate most of the responses from conductive ground, leaving responses from bedrock conductors, cultural features (e.g., telephone lines, fences, etc.) and edge effects. Edge effects often occur near the perimeter of broad conductive zones. This can be a source of geologic noise. While edge effects yield anomalies on the EM difference channels, they do not produce resistivity anomalies. Consequently, the resistivity channel aids in eliminating anomalies due to edge effects. On the other hand, resistivity anomalies will coincide with the most highly conductive sections of conductive ground, and this is another source of geologic noise. The recognition of a bedrock conductor in a conductive environment therefore is based on the anomalous responses of the two difference channels (DFI and DFQ) and the resistivity channels (RES). The most favorable situation is where anomalies coincide on all channels.

The DP channels, which give the apparent depth to the conductive material, also help to determine whether a conductive response arises from surficial material or from a conductive zone in the bedrock. When these channels ride above the zero level on the digital profiles (i.e., depth is negative), it implies that the EM and resistivity profiles are responding primarily to a conductive upper layer, i.e., conductive overburden. If the DP channels are below the zero level, it indicates that a resistive upper layer exists, and this usually implies the existence of a bedrock conductor. If the low frequency DP channel is below the zero level and the high frequency DP is above, this suggests that a bedrock conductor occurs beneath conductive cover.

The conductance channel CDT identifies discrete conductors which have been selected by computer for appraisal by the geophysicist. Some of these automatically selected anomalies on channel CDT are discarded by the geophysicist. The automatic selection algorithm is intentionally oversensitive to assure that no meaningful responses are missed. The interpreter then classifies the anomalies according to their source and eliminates those that are not substantiated by the data, such as those arising from geologic or aerodynamic noise.

Reduction of geologic noise

Geologic noise refers to unwanted geophysical responses. For purposes of airborne EM surveying, geologic noise refers to EM responses caused by conductive overburden and magnetic permeability. It was mentioned previously that the EM difference channels (i.e., channel DFI for inphase and DFQ for quadrature) tend to eliminate the response of conductive overburden. This marked a unique development in airborne EM technology, as DIGHEM is the only EM system which yields channels having an exceptionally high degree of immunity to conductive overburden.

Magnetite produces a form of geological noise on the inphase channels of all EM systems. Rocks containing less than 1% magnetite can yield negative inphase anomalies caused by magnetic permeability. When magnetite is widely distributed throughout a survey area, the inphase EM channels may continuously rise and fall, reflecting variations in the magnetite percentage, flying height, and overburden thickness. This can lead to difficulties in recognizing deeply buried bedrock conductors, particularly if conductive overburden also exists. However, the response of broadly distributed magnetite generally vanishes on the inphase difference channel DFI. This feature can be a significant aid in the recognition of conductors which occur in rocks containing accessory magnetite.

EM magnetite mapping

The information content of DIGHEM data consists of a combination of conductive eddy current responses and magnetic permeability responses. The secondary field resulting from conductive eddy current flow is frequency-dependent and consists of both inphase and quadrature components, which are positive in sign. On the other hand, the secondary field resulting from magnetic permeability is independent of frequency and consists of only an inphase component which is negative in sign. When magnetic permeability manifests itself by decreasing the measured amount of positive inphase, its presence may be difficult to recognize. However, when it manifests itself by yielding a negative inphase anomaly (e.g., in the absence of eddy current flow), its presence is assured. In this latter case, the negative component can be used to estimate the percent magnetite content.

A magnetite mapping technique was developed for the coplanar coil-pair of DIGHEM. The technique yields a channel (designated FEO) which displays apparent weight percent magnetite according to a homogeneous half space model.3 The method can be complementary to magnetometer mapping in certain cases. Compared to magnetometry, it is far less sensitive but is more able to resolve closely spaced magnetite zones, as well as providing an estimate of the amount of magnetite in the rock. The method is sensitive to 1/4% magnetite by weight when the EM sensor is at a height of 30 m above a magnetic half space. It can individually resolve steep dipping narrow magnetite-rich bands which are separated by 60 m. Unlike magnetometry, the EM magnetite method is unaffected by remanent magnetism or magnetic latitude.

The EM magnetite mapping technique provides estimates of magnetite content which are usually correct within a factor of 2 when the magnetite is fairly uniformly distributed. EM magnetite maps can be generated when magnetic permeability is evident as negative inphase responses on the data profiles.

Like magnetometry, the EM magnetite method maps only bedrock features, provided that the overburden is characterized by a general lack of magnetite. This contrasts with resistivity mapping which portrays the combined effect of bedrock and overburden.

Recognition of culture

Cultural responses include all EM anomalies caused by man- made metallic objects. Such anomalies may be caused by inductive coupling or current gathering. The concern of the interpreter is to recognize when an EM response is due to culture. Points of consideration used by the interpreter, when coaxial and coplanar coil-pairs are operated at a common frequency, are as follows:

1.	Channels CXP and CPP monitor 60 Hz radiation. An anomaly on these channels shows that the conductor is radiating power. Such an indication is normally a guarantee that the conductor is cultural. However, care must be taken to ensure that the conductor is not a geologic body which strikes across a power line, carrying leakage currents.

2.	A flight which crosses a "line" (e.g., fence, telephone line, etc.) yields a center-peaked coaxial anomaly and an m- shaped coplanar anomaly.4 When the flight crosses the cultural line at a high angle of intersection, the amplitude ratio of coaxial/coplanar response is 4. Such an EM anomaly can only be caused by a line. The geologic body which yields anomalies most closely resembling a line is the vertically dipping thin dike. Such a body, however, yields an amplitude ratio of 2 rather than 4. Consequently, an m- shaped coplanar anomaly with a CXI/CPI amplitude ratio of 4 is virtually a guarantee that the source is a cultural line.

3.	A flight which crosses a sphere or horizontal disk yields center-peaked coaxial and coplanar anomalies with a CXI/CPI amplitude ratio (i.e., coaxial/coplanar) of 1/4. In the absence of geologic bodies of this geometry, the most likely conductor is a metal roof or small fenced yard.5 Anomalies of this type are virtually certain to be cultural if they occur in an area of culture.

4.	A flight which crosses a horizontal rectangular body or wide ribbon yields an m-shaped coaxial anomaly and a center- peaked coplanar anomaly. In the absence of geologic bodies of this geometry, the most likely conductor is a large fenced area.5 Anomalies of this type are virtually certain to be cultural if they occur in an area of culture.

5.	EM anomalies which coincide with culture, as seen on the camera film or video display, are usually caused by culture. However, care is taken with such coincidences because a geologic conductor could occur beneath a fence, for example. In this example, the fence would be expected to yield an m- shaped coplanar anomaly as in case #2 above. If, instead, a center-peaked coplanar anomaly occurred, there would be concern that a thick geologic conductor coincided with the cultural line.

6.	The above description of anomaly shapes is valid when the culture is not conductively coupled to the environment. In this case, the anomalies arise from inductive coupling to the EM transmitter. However, when the environment is quite conductive (e.g., less than 100 ohm-m at 900 Hz), the cultural conductor may be conductively coupled to the environment. In this latter case, the anomaly shapes tend to be governed by current gathering. Current gathering can completely distort the anomaly shapes, thereby complicating the identification of cultural anomalies. In such circumstances, the interpreter can only rely on the radiation channels and on the camera film or video records.

MAGNETICS

The existence of a magnetic correlation with an EM anomaly is indicated directly on the EM map. In some geological environments, an EM anomaly with magnetic correlation has a greater likelihood of being produced by sulfides than one that is non-magnetic. However, sulfide ore bodies may be non-magnetic (e.g., the Kidd Creek deposit near Timmins, Canada) as well as magnetic (e.g., the Mattabi deposit near Sturgeon Lake, Canada).

The magnetometer data are digitally recorded in the aircraft to an accuracy of 0.01 nT for cesium magnetometers. The digital tape is processed by computer to yield a total field magnetic contour map. When warranted, the magnetic data may also be treated mathematically to enhance the magnetic response of the near-surface geology, and an enhanced magnetic contour map is then produced (editor - not given here). The passband components of the airborne data are amplified 20 times by the enhancement operator. This means, for example, that a 100 nT anomaly on the enhanced map reflects a 5 nT anomaly for the passband components of the airborne data.

The enhanced map, which bears a resemblance to a downward continuation map, is produced by the digital bandpass filtering of the total field data. The enhancement is equivalent to continuing the field downward to a level (above the source) which is 1/20th of the actual sensor-source distance.

Because the enhanced magnetic map bears a resemblance to a ground magnetic map, it simplifies the recognition of trends in the rock strata and the interpretation of geological structure. It defines the near-surface local geology while de-emphasizing deep-seated regional features. It primarily has application when the magnetic rock units are steeply dipping and the earth's field dips in excess of 60 degrees.

Any of a number of filter operators may be applied to the magnetic data, to yield vertical derivatives, continuations, magnetic susceptibility, etc. These may be displayed in contour, color or shadow.

VLF

VLF transmitters produce high frequency uniform electromagnetic fields. However, VLF anomalies are not EM anomalies in the conventional sense. EM anomalies primarily reflect eddy currents flowing in conductors which have been energized inductively by the primary field. In contrast, VLF anomalies primarily reflect current gathering, which is a non- inductive phenomenon. The primary field sets up currents which flow weakly in rock and overburden, and these tend to collect in low resistivity zones. Such zones may be due to massive sulfides, shears, river valleys and even unconformities.

The VLF field is horizontal. Because of this, the method is quite sensitive to the angle of coupling between the conductor and the transmitted VLF field. Conductors which strike towards the VLF station will usually yield a stronger response than conductors which are nearly orthogonal to it.

The Herz Industries Ltd. Totem VLF-electromagnetometer measures the total field and vertical quadrature components. Both of these components are digitally recorded in the aircraft with a sensitivity of 0.1 percent. The total field yields peaks over VLF current concentrations whereas the quadrature component tends to yield crossovers. Both appear as traces on the profile records. The total field data are filtered digitally and displayed as contours to facilitate the recognition of trends in the rock strata and the interpretation of geologic structure.

The response of the VLF total field filter operator in the frequency domain is basically similar to that used to produce the enhanced magnetic map. The two filters are identical along the abscissa but different along the ordinant. The VLF filter removes long wavelengths such as those which reflect regional and wave transmission variations. The filter sharpens short wavelength responses such as those which reflect local geological variations.

6.0 CONCLUSIONS AND RECOMMENDATIONS

This report provides a very brief description of the survey results and describes the equipment, procedures and logistics of the survey.

There are many anomalies in the survey blocks which are typical of massive sulphide responses. The survey was also successful in locating a few moderately weak or broad conductors which may warrant additional work. The various maps included with this report display the magnetic and conductive properties of the survey area. It is recommended that the survey results be reviewed in detail, in conjunction with all available geophysical, geological and geochemical information. Particular reference should be made to the computer generated data profiles which clearly define the characteristics of the individual anomalies.

Most anomalies in both areas are moderately strong and well- defined. Some have been attributed to conductive overburden or deep weathering, and others appear to be associated with magnetite-rich rock units. Zone 2 for the Unga block coincides with VLF anomalies which may reflect a fault or shear. Such structural breaks are considered to be of particular interest as they may have influenced mineral deposition.

The interpreted bedrock conductors defined by the survey should be subjected to further investigation, using appropriate surface exploration techniques. Anomalies which are currently considered to be of moderately low priority may require upgrading if follow-up results are favourable.

It is also recommended that image processing of existing geophysical data be considered, in order to extract the maximum amount of information from the survey results. Current software and imaging techniques often provide valuable information on structure and lithology, which may not be clearly evident on the contour and colour maps. These techniques can yield images which define subtle, but significant, structural details.

Respectfully submitted,

DIGHEM SURVEYS & PROCESSING INC.

David E. Pritchard

Geophysicist

DEP/sdp

A0568SEP.91R

APPENDIX A

LIST OF PERSOMNEL

The following personnel were involved in the acquisition, processing, inteX!pretation and presentation of data, relating to a DIGHEMIV- airborne geophysical survey carried out for Battle Mountain Exploration Company, on Unga Island and Popof Island, Alaska.

Steve Kilty	Vice President, Operations

Dave Pritchard	Survey Operations Supervisor

Maurie Bergstrom	Senior Geophysical Operator

Jordan Cronkwright	Second Geophysical Operator

Marcel Schubert	Pilot (Temsco Helicopters Ltd.)

Gordon Smith	Computer Processor

Dave Pritchard	I	nterpretation Geophysicist

Reinhard Zimmerman	Drafting Supervisor

Lyn Vanderstarren	Draftsperson (CAD)

Susan Pothiah	Word Processing Operator

Albina Tonello	Secretary/Expeditor

The survey consisted of 512 km of coverage, flown from June 20 to July 6, 1990.

All personnel are employees of Dighem Surveys & Processing Inc., except for the pilot who is an employee of Temsco Helicopters Ltd.

DIGHEM SURVEYS & PROCESSING INC.

David E. Pritchard

Geophysicist

DEP/sdp

Ref:	Report #568

A0568SEP.91R