In this primer we use the term electrical resistivity to refer both to the method and the property which is being measured. While this terms are commonly used, it is important to be aware that
- The term electrical resistivity can refer both to a complex property (meaning that it has both a real and imaginary part) or to the real part of that complex property. Or, for actual soils electrical resistivity will often be a complex property, and electrical resistivity instruments can measure either this complex property or only the real part of this complex property. A detailed discussion of this and of complex resistivity in general is given on this EPA website
- There are other alternative names which are used to describe the electrical resistivity method. Some of these names refer to methods which measure complex resistivity
- Some companies who collect and process electrical geophysics data refer to their approach by a name which is specific to their company to indicate some real or perceived difference between their approach and approaches by other groups. While there are a number of different ways in which data can be collected, processed and visualized it is important that the basic principles and underlying physics of electrical geophysical methods are the same for everybody. Claims of exclusivity should be carefully considered.
- In most cases electrical resistivity as a method refers to method where we have contact between the earth and the electrodes through galvanic coupling. However, a class of instruments and methods exist where we have capacitive coupling. Data from capacitive methods (which are especially appealing for land streamers and towed systems) can in general be processed is electrical resistivity data but subtle differences exist in processing and interpretation.
Subsurface Insights staff prefers the term electrical geophysical methods to describe all methods in which we measure electrical properties of the subsurface, however there are many other terms out there. A term which is commonly encountered is ERT (which is an abbreviation of Electrical Resistivity Tomography, a term which originated in the use of electrical geophysical methods in a borehole setting). In many cases ERT is know also used to refer to surface resistivity acquisition, and in general ERT has become a synonym for electrical resistivity.
In cases where the complex resistivity is measured we often talk about IP (Induced Polarization) methods or CR (Complex Resistivity) methods, where the difference is that IP methods measure complex resistivity in the time domain whereas CR methods measure complex resistivity in the frequency domain. However, the hardware is typically referred to as electrical resistivity instruments.
In addition to the different naming for the method there is some substantial variability in naming conventions used for different datasets. The naming conventions given below make sense for Subsurface Insights staff but many other conventions exist
- Resistivity measurement: A single four electrode measurement
- Resistivity dataset: A collection of resistivity measurements which is used to generate a single distribution of subsurface electrical properties
- Timelapse survey: A sequence of resistivity datasets
- Resistivity measurement file: A file produced by the acquisition system. A dataset can consist of multiple files, or multiple datasets can be contained in one file
- Reciprocal measurements: Two four electrode resistivity measurements in which the current electrodes in one measurement are used as potential electrodes in the other measurement. Reciprocal measurements should give the same apparent resistivity and comparing them allows for a rapid quality test.