Magnetic Methods
A Scintrex Envi-Mag proton-precession magnetometer in total-field mode allowed us to create accurate magnetic profiles of the two survey areas. We made two North-South survey lines at each of the locations, with about 300m spacing between lines, and stationing on each line about every 50m. Distances were not precise, and a combination of pacing and GPS distancing was used. In order to minimize error we took several magnetic readings at each station, until three readings were reasonably close together (usually within one nanotesla). On lines 1, 2, and 3 we ran a magnetic base station during the day, allowing later correction of the data for diurnal variations. No base station was used for line 4 because of difficulties getting the magnetometer to function correctly, but we attempted to take a few readings over the day with the roving magnetometer at the same location. There were several known and likely sources of error, though for the most part they did not seem to have a large impact on the results of the survey.
The large number of people that circulated through the magnetics crew led to poor quality control, and differences in the metallic objects and clothing worn by the surveyor could have introduced inconsistencies. External magnetic sources, such as power lines, bridges, and cars are also known or suspected to have influenced several of the readings. In fact, the magnetic nature of basalt in some locations returned a wide variety of readings over just a short distance.
Because of difficulties in downloading data from the magnetometers and errata in the base station data, base station correction could not be performed directly. Rather, diurnal variation was approximated by a series of linear trend lines, and corrections were made accordingly. Total discrepancy between the actual base-station data and the linear approximations was as much as +/- 3 nT in the worst cases. We did not correct line 4 for diurnal variation, because variation in base-station readings taken by the roving unit had a 10 nT mismatch compared with the diurnal variation observed on the other days. Since the magnetic anomalies that were the object of the survey occurred over a distance of a few hundred meters, all readings for a given anomaly were taken within a few minutes of each other. Therefore, though the overall dataset may have a discrepancy of about 35 nT over the course of the day, individual anomalies the should be almost unaffected by the lack of correction.
Initial data analysis and correction was performed in Microsoft Excel, the average of the readings at each station was used, and the base-station corrections were made. Magnetic profiles were created in Excel by plotting latitude and magnetic variability. Modeling was done with the GM-SYS magnetic modeling program. In theory, modeling was performed in by adjusting the properties of the layers in GM-SYS until they matched the shape and magnitude of the Excel profiles. In practice, only the shape of the profiles could be achieved through modeling due to time constraints. Figures 1 a and 1 b show that magnetic profile of lines 3 and 4 (at Hazen Flat) could have been created by a sedimentary basin, though quantitative values are not correct. Modeling of lines 1 and 2 (at Fernley) was not as successful, chiefly due to time constraints.
Figure 1a. Corrected observed magnetic profile from Hazen Flat.
Figure 1b. The shape of the GM-SYS model is a very close match to the observed magnetic profile’s shape.
Figure 2. Line 1 magnetic profile from Fernley.
Figure 3. Attempted Fernley model.
Figure 4. Another attempted Fernley model.