Long Valley Ground Magnetic Data Acquisition
The ground magnetic survey was conducted from July 27 to August 1,
2003. Data were collected along the jeep trails west of Highway
395 and east of the caldera’s topographic boundary (Figure 4) using a Geometrics G-858 portable
cesium
magnetometer at sampling rates ranging from
0.5 to 8 sec in the gradiometer mode, with the two sensors being
separated by a distance of 1 m. The G-858 has an operating range
from 17,000 nT to
100,000 nT. Positions were established using a
Garmin eTrex Vista GPS receiver, which was synchronized through the
RS-232 port of the G-858, thereby allowing simultaneous readings from
the GPS and magnetometer to be collected and stored in the internal
memory of the magnetometer. During the survey, the accuracy of
the
GPS ranged from 3 to 20 m, and, on occasion, the GPS lost communication
with satellites due to tree coverage. The G-858 has the capacity
to store 1 MB of data, which, at a sampling rate between 0.5 and 8
seconds in gradiometer mode and simultaneous collection of GPS data
allows for approximately 1 hour of data acquisition. Therefore,
profiles along the jeep trails that could be collected in 1 hour or
less were
chosen. Upon completion of a profile, the data were
transferred in the field to a laptop computer before the subsequent
profile was initiated. A Geometrics 856 (G-856) portable proton
precision magnetometer was used as a base station to monitor the
diurnal variations. The G-856 has an operating range of 20,000 to
90,000 nT. These data were collected at 60 second
intervals.
The base station was centrally located within the field area (N37º
42’ 27.6”, W118º 58’ 37.7, Figure 4)
at a location
free of
external noise. The G-856 experienced technical difficulties on
the afternoon of July 30, 2003 and, therefore, was not used on July 31
and August 1, 2003. The G-858 also encountered problems and, on
occasion, the reading in the top sensor was noisy (i. e. profile 16).
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