COMPLEX RESISTIVITY & HACSAMT PROGRAM
Section 10, Page 17 January 2003
10.6 3-POINT DECOUPLING
When operating in low resistivity environments with large dipole spacings, as is common with
dipole-dipole or pole-dipole surveys, electromagnetic coupling is often an overwhelming factor
in IP measurements. The Complex Resistivity (CR) program provides for automatic removal of
electromagnetic effects by using a 3-Point decoupling algorithm, which assumes that the IP
effect at very low frequencies is relatively independent of frequency. This routine works well in
moderately coupled environments. For extreme coupling environments (e.g., 10 ohm-meter
ground and using 300 m dipoles), we recommend that full frequency referenced CR be used to
permit more precise coupling removal.
Please note that 3-point decoupling values are easily contaminated by noise. The operator may
want to have the raw phase displayed during run-time. By pressing when RAWφ
φφ
φ is
displayed above the key, the default real-time display will be changed to the raw phase. The
operator can change between 3-point and raw phase by repeatedly pressing
.
The formula used for 3-point decoupling is as follows:
φ
3pt
= 1.875φ
1
- 1.25φ
3
+ 0.375φ
5
φ
1
= phase at the fundamental frequency
φ
3
= phase at the third harmonic
φ
5
= phase at the fifth harmonic
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