Induced polarization (IP) is a second-order resistivity measurement that quantifies the charge storage capacity of earth materials. IP data may be acquired in a variety of ways depending on instrument design, including time and frequency domains, with each approach having its own benefits and short-comings. IP data can be acquired as an addition to a resistivity survey. New methodologies have made it possible to acquire IP measurements using the same arrays, wires, and electrodes as a standard resistivity survey, making the setup and acquisition more cost effective.
Induced polarization and electrical resistivity are core services of hydrogeophysics Inc. We have applied these methods & services in cutting edge ways gaining new knowledge in technically challenging environments.
While originally developed for the prospection and characterization of mineral deposits, which represent well polarizable targets, the value of the Induced polarization method also has been recognized for near-surface studies in relatively low-polarizable, sedimentary environments. The development has become possible due to considerable improvements over the last decade in instrumentation, modeling, inversion techniques, and an increased understanding of the origin of IP effects. Promising applications of Induced Polarization methods and services are particularly seen in the rapidly emerging fields of hydrogeophysics and biogeophysics, including for instance the characterization of hydraulic properties or the monitoring of biogeochemical processes in the subsurface.
Time Domain Induced Polarization (TDIP)
Time domain Induced polarization consists of measuring the voltage decay after the cessation of transmitted current. The area under a designated portion of the decay curve can be integrated and offered as an IP measurement of chargeability. Alternatively, IP data can be given as discrete values along the decay curve at specified times. The voltage decay is always measured for both positive and negative polarities to avoid DC offsets due to self-potential and telluric currents. In order to make measurements comparable from one location to another or from one array to another, Newmont Mining Corporation developed a “standard” for (TDIP) measurements. Other standards have been offered, but the principal goal in making an IP measurement is to determine anomalous responses from a background.
Hydrogeophysics has over 20 years of experience acquiring, processing, and interpreting Induced polarization data for projects all over the world.
TDIP is particularly well suited for problems related to landfills, organic non-aqueous phase liquid (NAPL) plumes, and lithologic discrimination. Fortunately, the method has been adapted to acquire relatively high quality field data with existing multichannel resistivity systems, so that acquiring IP data can be inexpensive and straightforward. Below is an example of acquiring IP alongside resistivity using the Super Sting R8. A multichannel array of 168 electrodes at 3-meter spacing was placed in a complex geological terrain. Below the geophysical data are details revealing surface geology. The data show the diabase to be of high resistivity and low chargeability and the tuff to be of low resistivity and high chargeability.
Frequency Domain Induced Polarization (FDIP)
When measuring Induced polarization in the frequency domain (FDIP), a phase shift of the received voltage waveform relative to the transmitted waveform is recorded. The amplitude of the voltage waveform is used to calculate the resistivity. When both phase and amplitude measurements are made, the resultant value is referred to as complex resistivity. Furthermore, multiple frequencies can be acquired (practically over a range from 0.1 to 100 Hz), where both phase and amplitude are measured for all frequencies and is referred to as spectral IP (SIP).
HGI uses FDIP mainly for mineral exploration. Below is an example of FDIP data acquired for mapping a deep sulfide deposit. Data were acquired at 0.1 and 1.0Hz using the dipole-dipole array and inversion models produced a resistivity (ohm-m) and phase value (mrad). The near surface material is conductive and related to alluvium. At depth, bedrock shows up as resistive. Within the resistive material, IP targets are found in both the more conductive (center of line) and resistive (beginning of line) material; the latter is highlighted. By calculating the percent difference in phase shift between the frequencies, we can distinguish the importantance of the two IP targets that may be related to actual mineralization rather than simply lithology.