Electrical resistivity measurements at GLBRC: instrumentation, data collection, and processing

Diker, K., van Dam R.L., Bhardwaj A.K., Hamilton, S.K.

Presented at the GLBRC Sustainability Retreat (2010-02-10 to 2010-02-12 )

Root‐zone soil moisture is an important driver of processes in agricultural, hydrological, ecological, and climate systems, yet the spatial distribution of plant water use is often poorly characterized. To better understand the spatial and temporal variability in soil moisture for different bioenergy cops we installed electrical resistivity imaging (ERI) arrays at the Great Lakes Bioenergy Research Center (GLBRC). The electrical resistivity of a material depends on the chemical properties, the salinity, temperature, and especially the water content. Below all 10 vegetation plots in Block A of the GLBRC sustainability research site in Michigan east-west oriented arrays of 40 graphite electrodes have been installed. Each electrode is individually connected to underground cables that have take out points in the middle of the alleyways. Three temperature sensors have been installed at 5, 45, and 100cm in each vegetation plot and soil samples have been collected for laboratory analysis of the resistivity-water content relationship.

In 4-week intervals since May 4 2009, we have collected ERI data using a dipole-dipole configuration, leading to about 89 readings of apparent resistivity per vegetation plot. In order to properly convert ERI data into water content we have applied corrections for electrode depth and soil temperature. After inverting the data to obtain the lateral and vertical distribution of resistivity values, we applied lab-derived petrophysical relationships to obtain the distribution of water content.

A comparison of the ERI results with TDR soil moisture values shows good correlation between seasonal changes observed with both methods. These seasonal soil moisture dynamics reflect climatic conditions and plant water use. Our data show: a) lowest ERT- and TDR-derived soil moisture values are for Miscanthus and b) signs of seasonally variable (and plant-variable) in-plot moisture patterns. To reduce error propagation and to increase the accuracy of our analysis, sensitivity studies are needed on electrode depth variation, temperature correction, inversion settings, and moisture conversion.