Evaluation of direct push probes: Sensor interface analysis of DC resistivity probes

In near surface sedimentary exploration direct push technology has become popular for geophysical logging. The method is thought to have great potential to offer accurate information about the variability of physical parameters since the region of disturbed sedimentary formation due to probe injection is considered to be smaller compared to disturbances by classical borehole measurements. Technical and experimental design of direct push probes follow often those of established borehole probes. A systematic appraisal of the suitability of such tools for direct push logging procedure exposing the probes to a very high mechanical stress and rapid aging process has been missing in the past. Following a recently developed general framework for direct push system decomposition we analyze two different DC resistivity direct push probes with regard to their sensor interface. Simple laboratory experiments validate the setup of a numerical simulation of both probes revealing significant differences on the suitability of the chosen electrode arrangement. Differences in robustness with regard to surface abrasion result in changing probe responses which could, depending on the experimental design of the probe, cause resistivity value changes of almost 25% within approximately 15 operational hours, which leaves severe doubts about the suitability of established direct push logging probes for quantitative geophysical probing.