abem Terrameter LS, Instruction Manual

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ABEM Terrameter LS
91
11 Appendix B. Measurement Modes
In SP, resistivity and IP data acquisition it is essential to suppress various types of
noise, from sources such as electrode polarisation, telluric currents, power grid. The
Terrameter LS is equipped with sigma-delta AD-converters. These have built-in low
pass filtering and excellent noise suppression. Low pass filtering cannot, however, be
used in standard mode for IP measurements, since it would colour early time IP data
strongly. Instead proprietary signal processing is used in order to suppress noise while
at the same time resolving early time IP decay signals. The following parameter
definitions are used:
Parameter Description
Delay time Delay from current turn-on until measuring starts (for
instance 300ms)
Acquisition time Integration time (for instance 500ms = 25 samples @ 50Hz)
Number of IP windows No of IP windows (for instance 10)
IP delay time Delay from current turn-off until IP measuring starts (for
instance 10ms)
IP integration time(n) Integration time for IP time window (for instance 20ms).
The measuring cycles used in DC resistivity and time domain IP surveying eliminate
zero shifts by taking the average of data measured on positive and negative current
pulse. Furthermore, the measuring cycles are designed to minimise errors due to
variation in background potentials during the measuring cycle.
11.1 Self Potential (SP)
Self-potential measurements are done by simply integrating the input voltages over
the specified acquisition time interval, and if so selected stacking and averaging the
results. Care should be taken to select the base frequency and acquisition time so that
sufficient averaging and noise suppression is achieved.
In areas with 16 2/3 Hz (for instance railway power supply) it is essential to select an
acquisition time that is a multiple of 60 milliseconds to suppress such noise. This
applies to resistivity and IP data acquisition as well.
11.2 Resistivity (RES)
In the case of resistivity measurement the measuring cycle consists of a positive, a
double negative and again a positive current pulse (Figure 106). By averaging the
measured voltages all zero shift and linear drift during the measuring cycle is
eliminated.