Induced Polarization (IP)

Method

Introduction

An IP survey is produced by injecting current into the ground and measuring the change in voltage with respect to time (TD) or a lag in phase between the receiver and transmitter waveforms (CR). The voltage potential is measured between two receiver electrodes, and as the spacing between transmitter electrodes and the receiver pairs increases, the depth of investigation increases.

In simple terms, the target’s ability to store electrical energy after the current is turned off or after the polarity switches indicates the material is chargeable (ie sulfides, graphite, clays, or other alteration products).

  • Time-domain: The measured IP response is the secondary voltage decay produced by charging the interface of the target’s grain surface, which is dependent on the electrochemical properties of the target.

  • Complex Resistivity: Where there is no turn-off of the transmitter, the IP effect is observed as changes in shape of the receiver waveform, where information on the phase and magnitude is obtained. Higher IP amplitudes are associated with the chargeable nature of the target.

Through time-series processing, Zonge evaluates the nature of the IP response in both domains. The CR processing is in frequency-domain and able to apply decoupling methods in an attempt to remove the frequency dependent EM coupling responses. TD processing allows adjusting of the integration window which can provide information on the nature of early vs late time responses.

We obtain information on the variation of the resistivity (ohm*m) and chargeability (msec) or phase (mrad) with depth, and surveys are designed with various array configurations, such as Dipole-Dipole, Pole-Dipole, or as a 3D array.

Dipole-Dipole configuration example

Applications

The IP response is limited to specific materials and environments, where fluid-filled pore spaces may interact with metallic grains or clay. IP is most effective by comparing resistivity information to the chargeable responses of specific targets (ie disseminated sulfides).

IP is generally a follow-up survey from magnetic or gravity information that guides the orientation of IP lines, and is commonly deployed to prioritize drill targets.

  • Disseminated sulfide mineralization

    • Porphyry, Epithermal, Carlin Type, Orogenic, Uranium deposits

    • Cemented, massive sulphides are not viable IP targets, but associated disseminated zones of mineralization can produce an IP effect

  • Structures, depth to bedrock, and fault zones identified through complimentary resistivity data

  • Mapping of clay minerals or gravels in groundwater or mineral (alteration) applications.

  • Dissolved solids in groundwater (contaminants) and groundwater porosity

Examples of relative resistivity and IP responses from geologic settings.

Survey Design

ParameterDescription
DepthThe depth is based on the distance between current electrode and potential electrodes (n-spacing) and the observed signal-to-noise ratio.
Dipole LengthDistance between receiver electrodes (a-spacing). Larger dipoles increase depth but decrease resolution. Smaller dipoles are ideal for narrow vein targets and shallow features.
ScaleLocalized grid or profiles of electrodes. Lines often span 1.8 km or larger
ProductionProfiles are completed on a scale of days, where the dipole length, line length, and S/N are main contributions to production.
ProcessingData are collected as time-series, where the number of cycles may be adjusted based on data quality. 100% (CR) or 50% (TD) duty cycle data may be processed in either domain.

Let’s talk about it! Contact us today

Instrumentation

  • Receivers:

    • ZEN High-Res Receiver, 32-bit A/D’s using a 1024 hz sample rate
    • GDP-3224 Multi-Function Geophysical Receiver, 24-bit multi-channel

    · Transmitters:

    GGT-30, 30KVA or GGT-10, 10KVA

    · Generators: ZMG-30 or ZMG-9

Deliverables

  • Pseudosections of observed apparent resistivity and chargeability data
  • Results of two-dimensional inversions as sections of inversion chargeability/phase and resistivity versus depth
  • Plan maps of 2D inversion chargeability/resistivity results at selected elevations or depths.
  • 3D model of IP/Resistivity data at horizontal depth or elevation slices

Survey Life Cycle

  1. Survey design and planning of array type and line layout
  2. Electrode deployment and transmitter energization
  3. Data acquisition with real-time QC
  4. Processing of time-domain or phase-domain data
  5. Inversion of chargeability and resistivity
  6. Final interpretation and integration with geology

Case Studies and Resources