Induced polarization (IP) is the Earth’s capacity to hold an electric charge over time. IP measures the voltage decay curve after the injected current is shut off. The higher the IP, the longer over time the charge is held—IP decays over time, typically a few seconds but sometimes up to minutes, and will eventually disappear. IP is especially useful for mineral exploration applications.
How does induced polarization surveying work?
IP surveying is an exploration method involving the measurement of the slow decay of voltage in the ground following the cessation of an excitation current pulse (the time domain method)—AGI does not use the frequency domain method.
IP is a derivative of resistivity—in order to measure IP, first measure resistivity. IP is measured at the end of a resistivity cycle as follows:
First, measure resistivity.
- DC electric current is transmitted into the ground through two electrode stakes (called A and B) that are driven into the ground. The resulting electric potential field is measured between two other electrode stakes (called M and N).
- Raw measured data—i.e., apparent resistivity values—are inverted to produce a model of the true subsurface resistivity distribution.
- Correlations of the resistivity anomalies and specific targets can then be achieved by using ground truth sampling from drilling.
Then, add a time component to measure IP.
Whereas resistivity answers the question “What is the resistance to the flow of electric current?” IP answers the question “How long does the earth hold an electric charge?” To get IP, we measure the time component (you can use the same cable setup and the same instrument): Turn off the current injection, but keep recording the voltage between M and N for a few seconds. The decay in voltage of M and N over time is the IP.
Why is induced polarization surveying important?
The induced polarization method makes use of the capacitive action of the subsurface to locate zones where clay and other chargeable minerals are located within the host rock. With IP, you can tell very distinctly what is in the ground if it holds a charge because you end up with two maps—i.e., resistivity and IP—at the same time.
This is important for mineral exploration—identifying economically important minerals, including gold, copper, and silver. Early on, pyrite and massive sulfide minerals were deposited by hydrothermal fluids. These minerals have a huge IP signature, especially when disseminated as smaller grain in a rock matrix, which we can use when exploring for these minerals.
What is induced polarization surveying used for?
IP is used predominantly in the mining industry for mapping of disseminated sulfide bodies and other ore exploration, and it can also be used for environmental purposes and research. Here are two examples:
- Mapping gold deposits in the Yukon. Our client, Ground Truth Exploration, Inc., used the IP surveying method through AGI instruments and software to prove that gold deposits existed in areas of the Yukon Territory in Canada. The company had already ground truthed the gold deposit, but they wanted to prove that they could map any gold deposit locations in the Yukon frontier, and they used our tools to show how this type of mineral exploration could be done.
- Imaging an unintended hydrocarbon spill to help with cleanup. Our clients have used AGI tools to use IP to map a hydrocarbon spill. A superfund site consulting company used the SuperSting with resistivity and IP to successfully locate areas of zones contaminated with creosote in the near surface. Without drilling into the surface, they also imaged the clay and sand where the spill was located. With this data, they could assist with cleanup. The data was then ground truthed by using a backhoe.
What AGI tools are used for induced polarization mineral exploration surveys?
- SuperSting WiFi with IP.
- Non-polarizable electrodes. While you could use normal electrodes, we suggest using non-polarizable electrodes to improve the data quality.
- Cables. Dual mode cables are preferred, as they can transmit and collect IP-type data. However, you can collect on normal lines as long as you have a strong IP target.