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. Upon termination of the current, the induced polarization resulting from an applied voltage differential across the material, begins to decay. The rate of decay is characteristic for certain minerals. A second method, which our system utilizes, is to measure the phase shift in an alternating current. As the current alternates direction of applied voltage differential, the speed at which polarization reacts can be measured as the phase shift. The amount and frequency of a 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.

Instrumentation

We use a light weight frequency domain IP/Resistivity-Meter with transmitter currents of up to 100 mA with a voltage of 400V peak to peak. Despite the low transmission current the IP system delivers, reliable data up to a depth of approximately 50m (164 ft). This is possible due to distinctive features in the hardware such as: signal amplifier, electronic data-filtering and refinement, minimizing contact resistance between electrodes and ground material, increased data stacking, etc. This technological concept has been developed and refined in close collaboration by the measuring hardware manufacturer and Boreal GeoSciences.

Strategy

The concept of maximum phase shift IP is suitable for primary IP-prospecting that can be done on site following a Magnetics survey. Magnetic anomalies that are IP-active can be verified post-survey with the depth and extent of the potential ore bodies. A close meshed integration of Magnetics and IP is possible in the field, where onsite interpretation can encourage the completion or expansion of the survey plan.

Boreal GeoSciences' combined magnetic and shallow IP investigation is significantly more cost-effective than regular programs. Incompletely captured IP anomalies of greater depth can be selectively targeted for IP prospection using a system capable of higher signal output.

Application

For prospecting ore bodies, IP paired with Resistivity is commonly used as a primary geophysical survey. The sulfide minerals often associated with ores produce a strong IP signal.

When prospecting for placer, IP profiles can show subsurface stratification where sediments of different particle sizes can be discriminated (e.g. clay and gravel). IP data can also support bedrock identification in resistivity profiles. During 2D Resistivity surveys we simultaneously collect IP measurements, aiding in the interpretation of resistivity data.


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