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AEROMAGNETIC SURVEY IN THE PATAGONIA AREA ARIZONA

RFP 1434-CR-96-SA-O1247

OPERATIONAL REPORT

Project No. 96-A07-58

By

SIAL GEOSCIENCES INC.

April 1997

Table of Contents

1.0 INTRODUCTION

2.0 RESULTS OF THE COMPENSATION TEST FLIGHTS

3.0 EQUIPMENT USED

3.1 Airplane

3.2 Magnetometers

3.2.1 Aircraft magnetometer

3.2.2 Ground magnetometer

3.3 Positioning system

3.4 Ancillary equipment

4.0 DATA PROCESSING

4.1 Flightpath

4.2 Data compilation procedures

5.0 SURVEY PRODUCTS

1.0 INTRODUCTION

This report describes the data acquisition and processing of a high sensitivity aeromagnetic airborne survey carried out for the United States Geological Survey (USGS) by SIAL Geosciences Inc. Montreal Canada, in the vicinity of Nogales and Patagonia cities in Arizona. Survey flying commenced November 4, 1996 and was completed November 19, 1996. Final processed data, maps and the operational report were delivered at the beginning of April 1997. The survey parameters are described below:

• Line spacing: 250 m Line direction: east-west Control line direction: north-south Terrain clearance: 230 m
• Limits:

  -111.1629� 31.3601� Arizona - Mexico Border

  -111.1629� 31.7907�

  -110.6836� 31.7907�

  -110.6836� 31.3331� Arizona - Mexico Border

  -111.0737� 31.3320� Arizona - Mexico Border

2.0 RESULTS OF THE COMPENSATION TEST FLIGHTS

A compensation test flight was carried out, November 6, 1996.

* The amplitude was calculated after applying a highpass filter with cut-off of 6 fiducials.

Uncompensated FOM = 11.30

Compensated FOM = 1.78

3.0 EQUIPMENT USED

3.1 Airplane

The survey aircraft was Piper Navajo 310, registration C-FYTT. The aircraft was equipped with a magnetometer stinger and was specifically modified to reduce noise due to magnetic components and electrical currents at the magnetic sensor located in the stinger.

3.2 Magnetometers

3.2.1 Aircraft magnetometer

GEOMETRICS G822A Cesium split-beam total field magnetic sensor was used, with the following specifications:

• Inflight Sensitivity: 0.001 nT Heading error: 0.25 nT Gradient tolerance: 10,000 nT/in Noise envelope: < 0.5 nT
• Sampling interval: 0.1 sec.

3.2.2 Ground magnetometer

A GEM GSM-19 overhauser portable magnetometer, located at the base station, was used to monitor the fluctuations in the earth's magnetic field. The earth's magnetic field was measured every 3 seconds to record the diurnal activity. Data were recovered daily and the diurnal corrections were computed and applied to the survey data on-site in order to produce preliminary maps for quality control. The base station was located within 161 km of all survey points in an area of low magnetic gradient and free of cultural interference. The base station location is shown by a red dot in Figure 1.

The magnetic datum used for the ground station was: 48253.00 nT.

3.3 Positioning system

In flight positioning was sampled at a rate of 1 hertz using a TRIMBLE-4000SE post-flight differential GPS system in conjunction with PICODAS PNAV-2001 navigation console. The system enables data to be positioned to an absolute accuracy better than 10 meters. The system used a mobile receiver in the airplane and a reference receiver, located at the base station.

Ground clearance was sampled each second, using a King KRA 10A radar altimeter and a ROSEMOUNT barometric altimeter. The radar accuracy was of in and was limited to a maximum reading of 2500 feet.

Due to steep topography, especially in the northern part of the block, it was necessary to gain altitude well in advance of topographical highs, resulting in numerous areas where the ground clearance exceeded 2500'. Where this occurred the recorded values haved been replaced with dummy values (*).

3.4 Ancillary equipment

Digital data were acquired using a PDAS 1000 data acquisition system and were recorded on the hard-disk system every 0.1 second.

A Panasonic colour video camera and cassette recorder operating in NTSC format recorded an image of the terrain beneath the airplane. Line numbers, fiducial and raw GPS X and Y were superimposed on the video recording.

4.0 DATA PROCESSING

4.1 Flightpath

Flight path was recovered from the differential GPS X and Y data. It was verified daily in the field to plan reflights where needed.

4.2 Data compilation procedures

Both field and office systems use GEOSOFT software for data processing. In the field, the total magnetic field profiles were verified daily. Diurnal subtraction was carried out as a preliminary levelling stage, using a nominal base value of 48253 nT. Lines flown when diurnal variations exceeded the contract specifications were systematically selected as reflights.

At the office, the lag and heading errors were removed from the entire data set. Levelling of the survey data was often made difficult due to differences in altitude at line/tie intersections, caused by the steep and abrupt topography that made it very difficult to maintain constant ground clearance. Therefore intersection analysis was found to be unsuitable. However, a directional decorrugation filter supplied by GEOSFOT was used, and was found to give satisfactory results.

This filter rejects grid features perpendicular to line direction to a low-pass freqency equivalent to line spacing. To avoid filtering of geological features, the resultant levelling correction was then low-pass filtered to an equivalent of 15 seconds along line direction and limited to �12 nT.

The data were gridded using the bi-directional akima gridding algorithm supplied in the GEOSOFT software, with a grid spacing of 75 m. This technique was used in order to avoid undesirable artificial overshoots in the final grid that the minimum curvature

technique may have produced in areas of rapid change in gradient. Due to the fact that the akima spline method tends to produce sharper features near actual data points, thus resulting in a noisier grid, an additional hanning filter was applied to the final grid. Finally, the International Geomagnetic Reference Field 1995 was removed from the total magnetic field. The IGRF values was computed at the locations and elevations of the observed data points.

5.0 SURVEY PRODUCTS

The following products were delivered:

• Video tapes Operational report Flight path map (1:1 00000) Original digital recording of the diurnal magnetic field from the ground magnetometer, on CD-rom. Digital Grid Archives Geosoft format, on CD-Rom:

  • total magnetic field residual magnetic field
  • radar altimeter

• Digital profiles in XYZ GEOSOFT format including:

  • Line: line numbers* Fid: sample (fiducial) number Flight: flight number Date: year and julian date Time: UTC time (HHMMSS) GPSX: coordinates - UTM (in) GPSY: coordinates - UTM (in) Latitude: coordinates - latitude (degree) Longitude: coordinates - longitude (degree) GPSZ: barometric altimeter (in) Altrm: radar altimeter (in) Umago: uncompensted mag (nT) Mago: compensated original mag (nT) Baseb: diurnal recording (nT) Drift: diurnal correction (nT) Magf: levelled mag (nT)
  • Magigrf: residual mag (nT)

* The following nomenclature was used to indicate the flight direction: