The Geomagnetic Data

By Ray R. Collins

On the second field trip to Sithylemenkat Lake we took a magnetometer with us. We planned to measure the earth's magnetic flux from several hundred meters out in the lake (from a point south of the fault), to a point about that same distance north of the fault. These readings were for two purposes; one to verify the existence of the fault and the second to being mapping the magnetometer data for the lake to see if it showed evidence of faulting.

Fresh batteries were put in the magnetometer. The 3 meter long pole was assembled, the head was mounted vertically and the reader was attached. We started out in the lake, about 200 meters from shore and 300 meters from the airplane. Approximately every 50 meters we took a magnetometer reading. Due to fresh grizzly bear tracks we were carrying a rifle, so we were very careful to put the rifle down about 25 meters or more from the measurement location. Frances Nichols, who had completely cleaned out her pockets and had no metal on her other than a few things like metal zippers, held the pole while Ray Collins took the readings. To do a reading, Frances held the pole with the head; one end on the ground and the head pointed north (as per indicator arrow). Ray stretched the cable out to the east to the end (about 4 meters). Each reading was taken 3 times, and the results were averaged. If there was more than a 1 gamma difference between them a few extra readings were usually obtained. See field notes for specific information and a record of readings.

We took a path from out in the lake and climbed most of the way up to the "pass" (see Geological Map). When we were well past the fault we went directly east for about 1/3 km and then started readings again, headed back toward (approximately) where we hit the lake shore. Our final reading was in the same point where we made the eighth point.

Each location was determined using a GPS. These readings were recorded, using minutes and tenths of minutes only (since the degrees never changed). See field notes for the specific information and a record of readings. The GPS was allowed to sit for a number of minutes before the reading was taken after it settled down to help reduce error.

Results were somewhat disappointing. The GPS had such wide error bars that in places it looked like we were going backwards, even though we followed a nearly straight line (see graph 1). When graphing the magnetometer readings we assumed that our distance between readings was 50 meters on the outward journey and 70 meters on the inward journey, and graphed it in a linear fashion. This is likely to somewhat skew the data.

Both times when we went across the fault, which was expressed on the ground as a small dip in the terrain (running at 90° to the slope of the hill), there were anomalous (high) magnetometer readings. On the trek out the readings peaked at 59344.8 gamma. On the return trek it peaked at 61610 gamma. On the return trip when we found the peak we backtracked a little bit (25 meters) from the point where we got high reading and took another one; then we went forward 25 meters from the point where we got the high reading. Both these readings were markedly lower than the ourward journey. See graph 2.

The magnetometer readings at the eighth point (400 meters) were duplicated. These readings should have been the same, but they were not (by 2899.5 gamma).

The wide variation in data at the eighth point is very bad. It puts our entire data set in jeopardy, for it represents our minimum error. With error bars this wide (and probably wider) you could almost convert the readings to a straight line. Thus our conclusions can not be stated with much confidence.

One possible reason for the variation would be natural variation due to changes in the solar wind. To see if this were the cause we checked the magnetic variation for April 23^rd at College Alaska (using the USGS magnetic observatory page). Unfortunately the variation on that day was a great deal less than the variation we observed, so we must rule this out as a possibility.

Two other things may account for out data; first is operator error (though we have been unable to identify any specific cause, it should be noted that we are amateurs at using the instrument) and the second is instrument error. To date the instrument has not been checked out to verify operation.

In spite of the poor results it is encouraging that at the location we suspected the fault to be we did get anomalies in our data. So, while we are less certain than we would like, we can say that our data support a fault in that location.

Use of the data to support (or disprove) impact would not be advised.

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