# Electrical Resistivity Primer

## What is a geophysical image? A geophysical image is a visual representation of either geophysical data or an estimated discrete spatial distribution of a physical property obtained through the processing of geophysical data. A geophysical image can thus show one of two things

• Geophysical data
• A spatial distribution of physical properties resulting from the processing of geophysical data

In either case a geophysical image is a visual representation of discrete data (i.e. real or complex scalars or vectors) which is ordered in space and time and where the value of the data is represented as a color or grayscale value. This data can be 1D, 2D or 3D. Or, a geophysical image consists of discrete values (shown below in a 2 D grid), which are then mapped to colors to give the final image (as shown in cartoon form for a 2D case in the figure on the left).

As the focus of this primer is electrical geophysical methods we will focus here on the second case. However, one can wonder why we display geophysical data (the "raw product") at all instead of the spatial distribution of physical properties (the "end product"). The reason for this is twofold.

• In some cases processing is complex and the geophysical data itself is informative to show (for instance seismic reflection data, magnetic data or GPR data is often shown this way).
• The raw geophysical data can be rapidly displayed (typically in the field) and allows for qa/qc and a first assessment of subsurface properties.

If a geophysical image shows a spatial distribution of physical properties (for instance in the case of processed electrical resistivity data which is the focus of this primer) these properties should correspond to a certain part of the earth. This correspondence has several important attributes:

• The farther one is away from the geophysical instruments, the larger the grid will be on which these values are given.
• As discussed in the next section there is an inherent uncertainty in the values obtained from processing of geophysical data