Field-scale comparison of frequency- and time-domain spectral induced polarization

In this paper we present a comparison study of the time-domain (TD) and frequency-domain (FD) spectral induced polarization (IP) methods in terms of acquisition time, data quality and spectral information retrieved from inversion. We collected TDIP and FDIP surface measurements on three profiles with identical electrode setups at two different field sites with different lithology. In addition, TDIP data were collected in two boreholes using the El-Log drilling technique in which apparent formation resistivity and chargeability values are measured during drilling using electrodes integrated within the stem auger. The TD and FD data were processed and inverted with similar approaches. The IP voltage decays and complex impedance spectra were inspected quadrupole by quadrupole for outliers. In the inversion, the apparent resistivity values and the full IP decays (in TD) or the full complex impedance spectra (in FD) for all quadrupoles were inverted simultaneously in 2-D using amodified parametrization of the Cole-Colemodel. Furthermore, inversions of synthetic modelsmimicking the field situationswere performed. The comparisons reveal that TD and FD results are comparable not only qualitatively, but also quantitatively. Furthermore, the surface inversions are in agreement with the borehole results and lithology, within the resolution capability of the surface measurements. In particular, comparable spectral IP parameters were retrieved for both sites with both measurement approaches, with relaxation times between 10 -2 s and 1 s, suggesting a spectral coverage at least up to 100 Hz for both TD and FD measurements. However, for the employed data acquisition procedures and instrumentations, the TD measurements had an advantage in terms of acquisition speed and usable spectral acquisition range compared to the FD measurements. We conclude that the TD method, as the FD method, is a suitable tool to recover spectral IP information in the field, provided that the measurement procedures are planned accordingly and the subsurface IP response is in the spectral range covered by the measurements.