GDP-32
II
INSTRUCTION MANUAL
May 2002 Section 6, Page 28
A “good” resistance is under 2 kΩ. In some areas, however, resistances higher than 25 kΩ are
common. Over the Canadian Shield and in the high deserts of South America, contact
resistances can be 500 kΩ or higher. If high contact resistances are measured, check to make
sure that the pots are properly planted and are filled with saturated electrode solution (e.g.,
CuSO
4
.) In dry ground, it may be necessary to use two pots hooked in parallel to lower the
resistance. Zonge also manufactures potential electrode amplifiers (APOT) for use where the
contact resistance cannot be reduced sufficiently by passive means.
The status of CSAMT antennas and other types of active sensors can be monitored with the
CRES option. For the ANT/1, ANT/2 and TEM/3 antennas, the ON resistance varies from 0 to
200 Ω, and the OFF resistance is usually in excess of 2,000 Ω. Abnormally high CRES values
for battery-powered devices (antennas, APOTs, etc.) may indicate a dead battery or mechanical
problem.
6.4 SETTING THE POWERLINE NOTCH FILTER
There are several ways to determine if a powerline notch filter is necessary. One of the best
ways is to observe the cyclic noise on an oscilloscope and compare its amplitude to the signal
amplitude. Engage the filter if the powerline noise amplitude is noticeable. Refer to Section 5,
page 14 for a description of available powerline notch filters.
When an oscilloscope is not available, a technique used to estimate powerline noise is as follows:
1. Turn off the transmitter.
2. On the GDP-32
II
, enter the RPIP survey program and, in the Data Acquisition Screen,
select a high frequency (e.g. 512 Hz).
3. Set the Notch Filter OUT.
4. In normal data gathering mode, (i.e. with all dipoles connected) set the gains with the
auto-gain option, and note the G1 and G2 values (gain stages 1 and 2). This estimates the
ambient background noise with powerline noise.
5. Select a Notch Filter and reset all gains with the auto-gain option, and note the new G1
and G2 values. This estimates the ambient background noise without powerline noise.
6. If a significant difference in the G1 and G2 values is noted (i.e., a factor of 2 or more),
turn on the appropriate notch filters.
NOTE: Although a powerline notch filter may be required for only part of a survey, it is best to
complete the entire survey with the same notch filter configuration.
NOTE: If a survey is to be run with the powerline notch filters IN, the system MUST be
calibrated with the same powerline notch filters IN (see Section 6.1).
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