Geophysical Data
The IRIS DC-Resistivity instrument is combined with a switching unit that uses several multi-core cables to transfer current into metal electrode arrays, typically made of stainless steel and 50–100 cm long. Tens to hundreds of electrodes are typically used. The current is injected into one electrode and received at another. At the same time, voltages are measured at two other electrodes, and the potential difference between these electrodes is measured. Once current and potential difference are measured, resistance is calculated using Ohm’s Law. Finally, electrical resistivity -or its reciprocal quantity, electrical conductivity- is calculated, for this particular configuration of 4 electrodes.
Then, using industry-leading, free and open-source SimPEG software (Simulation and Parameter Estimation for Geophysics), data are analyzed to produce 2-dimensional and 3-dimensional visual representations of interpreted water sources. The software was initially created by members of UBC-GIF (UBC Geophysical Inversion Facility), many of whom continue as core developers on the project.
This process is known as geophysical inversion. Geophysical data is converted into a model of a physical property distribution in the subsurface, as the image below demonstrates:
For the DC-Resistivity method, electrical resistivity is converted into a map of apparent resistivity (an ‘average’ value), in the form of a 2-dimensional profile or a full 3-dimensional image. However, this diagram, called a ‘pseudo-section’, does not contain any reliable depth information, and so it cannot be interpreted as a true ‘image’ of the subsurface. To obtain a more reliable image of the subsurface, the data must be inverted, to obtain the model distribution of apparent resistivity.
In our post on “Interpretation of Geophysical Data to Find groundwater,” we provide a groundwater example of this entire process — including how the resulting inversion model can be interpreted to find the ideal locations at which to drill wells for groundwater.
