NATURAL SOURCE MAGNETOTELLURICS
Section 13, Page 23 May 2002
13.9 NOTES ON FIELD CONFIGURATIONS
When running multiple channel receiver systems, you must be very careful to avoid common
mode problems. Common mode effects are caused by lack of a reference voltage or level
(floating ground), or a reference level that exceeds common mode limits of the input amplifiers.
Common mode levels for the standard configuration of the GDP-32 are ±10 volts. With external
isolation amplifiers, this level can be extended to several thousand volts, but in exchange you
have to contend with higher noise and a lower overall frequency response.
The best configuration that we have found is to install a REFERENCE ELECTRODE (standard
copper/copper-sulfate electrode or equivalent), connected to analog ground (COM on the analog
side-panel) and the case ground (CASE GND on the side panel), positioned next to the receiver
and at least one meter distant from the nearest receiving electrode.
Another consideration is protection from static discharge and nearby lightning strokes. This
protection is maximized by connecting the case ground to the REFERENCE ELECTRODE as
well.
Additional protection in lightning-prone areas can be afforded by using a galvanized iron plate
(or equivalent) as a REFERENCE ELECTRODE. This plate should be buried close to the
receiver in a hole that has been well watered and the soil mixed to make good mud contact with
the plate. Typical size for the plate would be 30 by 30 cm.
We have found that for most environments, the best noise rejection is obtained by connecting the
analog ground (COM) to the case ground (CASE GND) on the analog I/O side panel.
The figures at the end of this chapter provide examples of receiver connections using the
REFERENCE ELECTRODE or REFERENCE POT connected to both analog ground (COM) and
case ground (CASE GND).
Note: For best results for natural source data acquisition, always use an external signal
conditioner with programmable gain and frequency band limiting capability, such as the SC-8.
Provision is made in Menu 3 to enter external gain factors to be included in the calculation of
apparent resistivity.
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