Induced Polarization (IP)

Method

One way to measure IP is to inject direct current into the soil. The induced polarization of the ground materials is the result of electrochemical processes. When the current is turned off it takes time for the induced voltage, to decay. The rate of decay is characteristic for certain minerals. Another method is to measure the phase shift in an alternating current, the amount and frequency with the maximum phase shift is characteristic for certain particle sizes and mineral compositions.

The measured data is then collated into a 2D-profile which allows us to draw spatial, chemical and textural conclusions about the subsurface conditions.

Technological Concept

We use a light weight frequency domain IP/Resistivity-Meter with transmitter currents of up to 100 mA, frequencies 0.26 to 30 Hz and 400V peak to peak. Despite the low transmission current the IP-system delivers reliable data up to a depth of app. 120m. This is possible due to distinctive features in the hardware, like the signal amplifier and electronic data-filtering and refinement, as well as a special data acquisition concept, consisting of minimizing the contact resistance between electrodes and ground material to less than 1kilo Ohm and increased data stacking etc.. This technological concept has been developed and perfected in close collaboration by the measuring hardware manufacturer and Arctic Geophysics.

Strategy

The described IP-concept can be put into practice by a two men crew in the field. The crew produces 1120m long IP-profiles with good to excellent data for a depth of 0m to app. 120m every day, as long as there are cut lines. The price for such an IP-investigation lies at 20-30% of conventional IP-measurements with depths of 200-250m - same line meters per day. This cheap and reliable IP-concept is suitable for primary IP-prospecting that can be done on site immediately following the Magnetics survey by the same crew. It can immediately be checked if the magnetic anomalies are IP-active as well as the depth and extent of the potential ore deposit. A close meshed integration of Magnetics and IP is possible in the field. Promising measuring results encourage the completion or even the flexible expansion of the survey plan; less promising data allow an early termination of the prospection with a smaller loss of investment.

This concept of a combined magnetic and shallow-IP investigation is significantly more cost-effective than regular programs and is very much appreciated by our customers. Incompletely measured IP-anomalies of great depth can be selectively targeted for an IP-prospection with higher signal output. The total effort is smaller and the secondary survey is based on the result of the primary investigation.

Application

For localising ores IP is an excellent method, together with Resistivity. The sulfide minerals, which are often associated with ores, produce a strong IP signal.

When prospecting for placer IP-profiles can show the subsurface stratification, where sediments of different particle sizes can be discriminated, for example muck (clay) and gravel.