Guard resistivity logs (also known as a Laterolog 3) were developed in ~1951 to provide better formation resistivity measurements in salty borehole fluids common in the petroleum industry. Saline borehole fluids adversely impact Normal Resistivity measurements, because the current produced at the tool preferentially flows through the electrically conductive fluid in the borehole and not into more resistive fresh water formations. (Note: induction logs were developed to evaluate formation resistivity in saline formations.)
Guard resistivity logs utilize symmetric large bucking or ‘Guard’ electrodes on either side of a much smaller current electrode to focus the current into a large thin disc. This focused current disc is much less impacted by the affects of borehole fluid. Multiple electrode designs have been developed for the petroleum industry to investigate different depths in the formation. The non-petroleum industry commonly uses only a 3-electrode or Short Guard measurement.
Typically the term Guard refers to the length of bucking electrodes (either 8 or 16”). The thickness of the centered current electrode determines the thickness of the current disc and resulting bed resolution.
There is also a micro-Guard measurement (found on our density tool) where the electrode array is shorter, and the electrodes are pressed up against the side of the borehole wall, thus reducing borehole affects from this very short electrode array.
This focusing of current leads to higher vertical definition of thin-bedded units and fractures than Normal Resistivity measurements, with the micro-guard having greatest vertical resolution. The Guard resistivity logs’ higher vertical definition has generated wide spread applications in the water well industry in identifying thin-bedded sands and fractured rock applications.
Guard logs aren’t as susceptible to borehole effects as a regular E-log would be. These tools are able to define 1” to 2” thick bed differences depending on the electrode size and style. The Guard measurement typically appears to be a very nervous measurement as it reacts to very small changes in very short depth intervals. The Guard Resistivity is a quantitative measurement.