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 decoupling methods can be applied 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.
Applications
IP is most effective when chargeability can be compared directly with resistivity to separate conductive clays, altered zones, and sulfide-bearing targets. It is commonly used after magnetic, gravity, or geologic mapping has defined preferred survey orientations.
- Disseminated sulfide mineralization in porphyry, epithermal, Carlin-type, orogenic, and uranium systems
- Structures, fault zones, and depth-to-bedrock features that also produce resistivity contrasts
- Mapping of clays, alteration, and gravel horizons in mineral and groundwater investigations
- Delineation of dissolved solids, porosity changes, and other hydrogeologic variations
Survey Design
| Parameter | Description |
|---|---|
| Depth | Depth of investigation depends on electrode spacing, n-spacing, ground resistivity, and the observed signal-to-noise ratio. |
| Dipole Length | Receiver dipole length controls the balance between depth and resolution. Smaller dipoles are useful for shallow or narrow targets, while larger dipoles improve depth coverage. |
| Scale | Surveys may be acquired as profiles, localized grids, or larger 3D layouts. Individual lines commonly extend to 1.8 km or more. |
| Production | Daily production depends on dipole length, line length, terrain, and target signal strength. |
| Processing | Time-series data can be processed in time-domain or complex resistivity workflows, with cycle counts and windows adjusted to site noise and survey objectives. |
Instrumentation
- Receivers: ZEN High-Res Receiver and GDP-3224 Multi-Function Geophysical Receiver
- Transmitters: ZT100, GGT-30 and GGT-10
- Generators: ZMG-30 and ZMG-9
Deliverables
- Observed apparent resistivity and IP data.
- Results of 2D and 3D inversions
Case Studies and Resources
- Zonge, K., J. Wynn, and S. Urquhart, 2005, Resistivity, induced polarization, and complex resistivity, in D. K. Butler, ed., Near-Surface Geophysics, SEG Investigations in Geophysics Series No. 13, Chapter 9. http://dx.doi.org/10.1190/1.9781560801719.ch9
- Contact Zonge for IP project examples and references relevant to your target style and survey scale.