Mapping changing resistivity in the sub surface
As resistivity is a “galvanic” method, it requires a physical electrical connection to the ground. This normally means that steel electrodes have to be pushed into the material under investigation, although it is possible to use plate electrodes on some hard surfaces. Two electrodes are used to inject current (“current electrodes”), and a minimum of two electrodes are used for voltage readings (“potential electrodes”). Knowing the injected current and the measured voltage, the resistance of the ground can be calculated.
The resistance value is then recalculated to apparent resistivity. The apparent resistivity is a result of current flowing through all of the ground beneath the electrodes rather than just information from a discrete point or layer. These values, on their own, will not be suitable for making an interpretation and thus a further processing step, called inversion, is required. In this process, software is used to find a model of resistivity distribution that would produce the same result as the data recorded in the field. The resistivity values within the returned model will form the basis for an interpretation.
In hydrogeophysical investigations such as groundwater mapping, clay (low resistivity) can be distinguished from sand (high resistivity), or salt water (low resistivity) from fresh water (high resistivity), or wet sand (lower resistivity) from dry sand (high resistivity).
However, the resistivity to material translation is non-unique and the resistivity span of the various geological components overlap, thus a certain level of ambiguity may have to be dealt with. Also, completely different geological layers in completely different geological scenarios can have the same resistivity, hence some prior knowledge of the geological setting from local knowledge, geological maps and / or boreholes must always contribute to a geological interpretation of a geophysical survey.