FIELD RESULTS WITH DOWNWARD CONTINUATION TECHNIQUE IN INDUCED POLARIZATION PROFILING USING POINT ELECTRODES1

The method of downward continuation is well known to those working in gravity, magnetic, SP and low-frequency electromagnetic exploration. It is demonstrated that the method of continuation can also be usefully employed in the interpretation of induced polarization gradient profiling using point electrodes to determine target depth. The apparent resistance R_a and chargeability M_a measurements obtained with point electrode excitation of the ground have been used to compute the values of (R_a)_l and (M_a)_l that would be obtained with a linear array. Continuation of the apparent polarizability profile thus obtained with the linear array gives a value for the depth of the target which agrees closely with that obtained by the continuation of the SP profile. On the other hand, continuation of the profile of the secondary transient signal (V_S)_L alone, yields a depth of the target which is in agreement with the borehole information. However, it is seen that the secondary transient voltage profiling response splits into two anomalies which fall on either side of the SP and/or (M_a)_l anomaly centre, and does not coincide with that of the latter.