Survey number: 165 (Pat Hill number)
Name: Naval Pet. Reserve No. 4
Date flown: 1945 and 1946
Company: US Navy
Flight specs: 2-4 mi, NE-SW, 1000 ft above ground.
Instrument: see below

Digitized by: Esther Castellanos, 6/95
Map scale: 1:500,000 

Datum: DGRF corrected (see USGS Open-file 85-835 for details)

Data format: Ascii format, XYZ records 
             Longitude (degrees), Latitude (degrees), Total Field (nT)
             written with FORTRAN format: (f10.5,x,f10.5,x,f8.1)
             Residual grid of Tien Grauch converted to XYZ points

----------------------------------------------------
NPRA data prepared for open-file release by V.J.S. Grauch
Released as USGS Open-file 95-835
----------------------------------------------------
Aeromagnetic map of Naval Petroleum Reserve No. 4, Alaska

Digitized by Esther Castellanos, June 1995
(to update digitized version previously included in the Decade of
North American Geology - DNAG - magnetic compilation of Godson).

Digitized from map printed on linen (from Pat Hill)
Map labeled as follows:

Aeromagnetic Map of Part of Naval Petroleum Reserve No. 4 and Adjacent
Areas, Alaska

Prepared in cooperation with the office of Naval Petroleum and 
Oil Shale Reserves

Alaska General Report No. 10, Plate 1

magnetic base relative to arbitrary datum
scale 1:500,000
contour interval 10 gammas
1947

Area inside heavy boundary (center of map) flown 1945, compiled in
1945-46.  Area outside heavy boundary (borders) flown in 1946, compiled
in 1946-47.

Maximum and minimum intensity values given on map.

This aeromagnetic map was prepared in 1946 and 1947 as a reconnaissance 
aid to geological and geophysical exploration for petroleum in northern Alaska.
It was originally released in restricted reports to the U.S. Navy,
which has now approved publication in the present form.

Field data were measured with an An/ASQ-3A magnetometer installed in a PBY-5A
airplane flown 1,000 feet above the ground using a radar altimeter
for elevation measurement.  Operation of the aircraft was by the
U.S. Navy.  Traverses were made two to four miles apart as shown on the map.
U.S. Army Trimetrogon photographs were used for pilot guidance, and
final plotting was done on 1:500,000 scale World Aeronautical Charts.
A ground magnetometer station was maintained at Point Barrow to detect
magnetic storms, which occasionally prevented aeromagnetic surveying.

The magnetic anomalies appearing on the map are attributed mainly to 
susceptibility changes in the crystalline basement rocks rather than to
basement topography.  A good correlation between the magnetic map and 
subsequent gravity surveys exists.

Note (from note attached to map)

Aeromagnetic data are obtained and compiled along a continuous line,
whereas ground mangetic surveys are made at separate points.  Errors within
the normal limits of any magnetic measurement may cause slight
discrepancies between flight lines in an aeromagnetic map,
which would be more obvious than similar discrepancies between 
points in a ground magnetic map.
For this reason as much care should be exercised in evaluating
magnetic features that appear as elongations along a single 
aeromagnetic traverse as in interpreting an anomaly indicated 
by a single ground station.
All flights at 1,000 feet above ground.

(map text entered by Rick Saltus, 22 June 1995)