Binning Method for Mapping Irregularly Distributed Continuous Resistivity Profiling Data onto a Regular Grid for 3-D Inversion

Abstract

Continuous resistivity profiling (CRP) is an emerging direct current (DC) resistivity method that is increasingly used in shallow fresh water and marine environments to support hydrogeological and near-surface geophysical studies. CRP is popular because it allows efficient data acquisition. However, it is generally not possible to precisely control the spacing and orientation of profiles during CRP data acquisition, resulting in irregularly distributed data. For 2-D profiles, the irregular distribution is handled by extracting relatively straight segments for inversion and using either interpolation or irregular element widths in the inversion process to account for unevenly spaced data along the profiles. Three-dimensional (3-D) inversion of CRP data sets is more challenging because most commercial inversion codes are applicable only to parallel or sub-parallel survey lines, requiring data on a regular grid. In this paper, we develop a technique for binning irregularly distributed CRP data onto a regular grid, suitable for most 3-D inversion codes. We evaluate the method on synthetic data sets, as well as field CRP data sets collected from two water reservoirs on Maui Island, Hawaii. Numerical results of the synthetic data inversion show that 20% and 10% model misfit between a “control group” and binned data can be achieved with data densities of 5 and 15 measurements in each bin, respectively. Also, the 3-D inverted resistivity volumes of the binned field CRP data sets show good agreement with independently collected 2-D profiles, resolving the same subsurface features.