An Unexplored Realm in the Heartland of the Southern Gulf ... - Famsi

The magnetometer was set to the base station mode to check for possible natural
or artificial interference. Once conditions were found to be acceptable, the G858 was set
to the Survey Mode and data were acquired in the continuous recording setting; this high
sampling rate allowed the operator to survey an area at a rapid pace (Geometrics
2001:28-32). One magnetic storm occurred during the survey, which was quickly
detected and the affected area was resurveyed. The overall survey area was divided into
manageable segments by using the eight contemporary ejido fields as individual sectors.
They were identified accordingly and each field was subdivided into 100 m east-to-west
segments. These segments were traversed in a generally north-to-south direction at 2 m
line intervals, with locational marks taken every 2 m as well. Upon completion of a
transect, a perpendicular survey was conducted at 5-15 m intervals to permit a cross
verification of readings and location. Data collection readings were checked at various
intervals during the day. Each evening, data from the GPS unit and the magnetometer
were downloaded to the project computer and checked for completeness and coverage.
Electroconductivity surveys were conducted across the areas of detected magnetic
anomalies to determine if they may have been caused by metal objects. The terrain
conductivity surveys were made using a Geonics EM-31 and ground conductivity
measurements ranged between 5 and 14 millisiemens per meter (mS/m), a scale
consistent with sand, silty sand, and loam (Bevan 1998). Metal objects would be expected
to generate in excess of 200 mS/m (Peace et al. 1996:7); therefore, metallic materials are
not believed to be the cause of the magnetic anomalies.
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