3.5 Pulse Duration

Some time-domain instruments allow the user to select the duration of the current injection. Commonly, pulse duration varies from 250 milliseconds (ms) to several seconds. Lower pulse duration results in shorter data acquisition time, but longer pulse duration may be key to achieving steady state voltages and thus data more consistent with the physics assumed for analysis (e.g., the Poisson Equation). Similar issues arise in hydrogeology when performing hydraulic analysis based on steady-state assumptions (e.g., Thiem-Equation based analysis), where pumping continues until a quasi-steady state is achieved and then measurements are made. Otherwise, transient analysis is necessary, for example, Theis-Equation-based analysis in the groundwater analogy and IP in electrical imaging. Pulses on the order of 250 ms may be acceptable when exclusively collecting ER data in conductive, low-clay media. In the presence of clays and other chargeable media, however, longer durations may be required to achieve equilibrium voltages. Acquisition of IP datasets generally requires longer pulse durations (1 s or greater is recommended) so that the sufficient discharge (or charge up) occurs to be reliably recorded. As an approximate guide, the acquisition of high-quality IP data is likely to increase the survey time two to three-fold over acquisition of only ER data. The length of the pulse duration can be varied, and surveys repeated, to determine the minimum duration necessary to achieve good data.