Physical Property Laboratory Measurements

IP and Resistivity

The induced polarization (IP) response and resistivity determinations performed in the physical property laboratory are made in the conventional time domain mode of operation. The IP responses of these samples are determined utilizing a technique developed by Newmont Exploration, Ltd. Using an 8.0 second period (2 seconds on positive, 2 seconds off, 2 seconds on negative, 2 seconds off), the IP response is measured during a window of .45 to 1.1 seconds during the off-time.

These two procedures are performed simultaneously using a GDP series Geophysical Data Processor, and a constant current transmitter (as low as 100 nanoamps). IP measurements are made in the time domain for a complete cycle of 8.0 seconds (two transmitter pulses) averaged over 16 cycles. The reported chargeability data may be transformed to the normalized standard Newmont Exploration, Ltd. induced polarization unit (M331) by multiplying by 1.53. The resistivity values are in ohm-meters.

The procedure requires specially prepared cores, bicylindrical cores or cubed samples. Moisture saturation under vacuum is required for reliable data. If the received samples are moderately dry, such as surface samples, vacuum impregnation with distilled water is performed before determination of the electrical properties. As a consequence, the reported resistivities for these samples may not be truly representative of the rock in situ but will depend on the amount of water ingested into the rock pores. If the samples are saturated with interstitial water upon receipt they may be run without impregnation with water.

Rock resistivities measured by laboratory techniques are normally not indicative of the macroscopic resistivities, inferred from surface resistivity measurements. Normally, laboratory resistivities are much higher than the observed field resistivities. Laboratory measurements on small core samples do not adequately sample the large volume average resistivities which can be grossly influenced by fractures and faults containing mineralized solutions. Fresh drill core samples which have not been exposed to the air or elevated temperatures will sometimes yield representative rock resistivities in the laboratory.

The induced polarization values determined in the laboratory are not as strongly affected. Laboratory measurements on small core samples are often more representative of the macroscopic induced polarization responses and should normally correlate very well with surface measurements.