Step‐on versus step‐off signals in time‐domain controlled source electromagnetic methods using a grounded electric dipole

Abstract

The time-domain controlled source electromagnetic method is a geophysical prospecting tool applied to image the subsurface resistivity distribution on land and in the marine environment. In its most general setup , a square-wave current is fed into a grounded horizontal electric dipole, and several electric and magnetic field receivers at defined offsets to the imposed current measure the electromagnetic response of the Earth. In the marine environment, the application often uses only inline electric field receivers that, for a 50% duty-cycle current waveform, include both step-on and step-off signals. Here, forward and inverse 1D modelling is used to demonstrate limited sensitivity towards shallow resistive layers in the step-off electric field when transmitter and receivers are surrounded by conductive seawater. This observation is explained by a masking effect of the direct current signal that flows through the seawater and primarily affects step-off data. During a step-off measurement, this direct current is orders of magnitude larger than the inductive response at early and intermediate times, limiting the step-off sensitivity towards shallow resistive layers in the seafloor. Step-on data measure the resistive layer at times preceding the arrival of the direct current signal leading to higher sensitivity compared to step-off data. Such dichotomous behaviour between step-on and step-off data is less obvious in onshore experiments due to the lack of a strong overlying conductive zone and corresponding masking effect from direct current flow. Supported by synthetic 1D inversion studies, we conclude that time-domain controlled source electromagnetic measurements on land should apply both step-on and step-off data in a combined inversion approach to maximize signal-to-noise ratios and utilize the sensitivity characteristics of each signal. In an isotropic marine environment, step-off electric fields have inferior sensitivity towards shallow resistive layers compared to step-on data, resulting in an increase of non-uniqueness when interpreting step-off data in a single or combined inversion.

Electrical Resistivity Anomalies Offshore a Carbonate Coastline: Evidence for Freshened Groundwater?

“…Data were acquired along five profiles labelled lines 2, 5, 6, 8, and 9 in Figures 1 and 2. Following the shallow-water sensitivity considerations of Haroon et al (2020), only step-on data were processed at stationary waypoints marked by black triangles in Figure 2. Data errors are quantified by stacking repeated step functions at each waypoint resulting in a time-dependent noise model that varied each day of the survey and also for each receiver (presumably due to electrode noise).…”

Saturation sensitivity and influencing factors of marine DC resistivity inversion to submarine gas hydrate

“…Fortunately, the resistivity of submarine gas hydrate is significantly different from that of marine sediments (Key, 2012;Attias et al, 2020;Cook et al, 2020;Haroon et al, 2020;Liu et al, 2020;Schwalenberg et al, 2020;Duan et al, 2021;Zhang et al, 2021). Collett and Kuuskraa (1998) analyzed the concentration of natural gas hydrate used by Archie equation.…”