OEMG Digital Geophysics for Land & Freshwater

 Principles of Operation

Both the acquisition and processing methods of OEMG Digital Geophysics have been modified from traditional land based resistivity methods. In traditional land-based resistivity surveys, an electrical current is injected into the sub-surface by means of two current electrodes or a Current Electrode Pair (CEP). The voltage gradient associated with the electric field of the injected current is measured by a Voltage Electrode Pair (VEP) placed between the CEP (Figure 3, left). Based then on the values of current and voltage measured between the VEP, the average or Calculated Resistivity (CR – measured as Ohmmeter (Ohmm)) for the volume of subsurface between the VEP is determined to the limit of penetration. Penetration is largely determined by the distance between the CEP. Therefore, multiple CEPs are placed at increasing distances about the VEP to cover a range of depths. The result is a field curve (Figure 3, right) that reflects the changing CR from each CEP. In this example, the CR for CEP 1 (Point 1) has returned a relatively low CR that rises quickly (points 2 to 3, etc). This is typical of a thin layer of soft sediments overlying rock. The above described field curves are then the basis for a qualitative assessment of the sub-surface geology.

Principles of Vertical Electrical Sounding – On land. Left: a typical land base setup for a resistivity survey. The VEPs are placed between the CEP. Current and voltage measured at the VEP is used to derive the Calculated Resistivity (CR). Right: a typical resistivity curve for a given injection of current as a function of CEP (X axis) and CR (Y axis).

The CR of a geological structure depends on its porosity, water saturation and the water resistivity. Gravel usually has a lower porosity than sand and its resistivity thus is higher. Clay with generally very high porosities shows very low resistivities. Conversely, solid rock has a low porosity resulting in very high resistivities. Every geological structure therefore has a unique resistivity.

Acquisition is then tailored to the requirements of the surveys, for example foundation surveys that require an understanding of the deeper geology, more electrodes are be placed for each shot. For a shallow survey of an aquifer or utilities or road alignment, less electrodes can be placed for each shot, meaning data can be acquired at a faster rate.