Continuous Resistivity Profiling in Shallow Marine and Fresh Water Environments

Abstract

In this paper, we describe an instrument system for performing continuous resistivity profiling in shallow freshwater and marine environments. Using a streamer cable containing 9 electrodes, the system continuously samples the dipole-dipole resistivity at n-spacings 1 through 6. The system can be installed aboard a variety of small inboard or outboard powered vessels in a few hours. Hand-held or marine GPS units provide location information that is recorded by a laptop computer. With this system, up to 40 line-km/day of dipole-dipole data have been collected. The resistivity data are merged with the GPS positions as a post-processing step. The final step in the post-processing is the inversion of overlapping segments of each profile using a 2-D smooth model. The inversions provide high resolution images of the geoelectric cross-section. The depth of investigation ranges from 20-30 m, with a 10 m dipole spacing. Over the last 4-years, we have performed surveys on the Ohio River, near Louisville, KY, on tidal estuaries and bays along the Atlantic coast in Delaware, Maryland, Virginia, and North Carolina, and in Tampa Bay, Florida. Data from these surveys will be used to illustrate the final deliverable from a survey.

Trends in waterborne electrical and EM induction methods for high resolution sub‐bottom imaging

“…Snyder et al () discussed technical issues involved in the design of a waterborne resistivity imaging system including the effect of water depth on measurements made with the floating dipole‐dipole electrode configuration. For cases where the sub‐bottom is 3 to 5 times more resistive than the water layer (typical of sands or silts saturated with the overlying water) they show that measured apparent resistivities will be at least 50% of the true sub‐bottom resistivity provided that the dipole length is at least twice the water depth.…”

“…Sensitivity to changes in sub‐bottom resistivity increases with the transmitter‐receiver dipole offset or . Based on these 1D modelling results and signal‐to‐noise considerations, Snyder et al () designed a dipole‐dipole array having six 10 m potential dipoles with the expectation that it would typically work effectively in water depths of up to 5–10 m.…”

“…Towed resistivity (continuous resistivity profiling) surveys have also been applied in recent years to investigate surface water/groundwater interactions that impact on the quantity or quality of water available for drinking and irrigation. The towed resistivity system described by Snyder et al (), for example, was designed to identify potential recharge areas (areas free of clay aquitard) to an aquifer underlying the Ohio River that supplied water to the city of Louisville, Kentucky, USA (Snyder and Wightman ). Allen and Merrick () and Kelly et al () described the use of towed resistivity arrays to assess the connectivity between surface water and groundwater beneath rivers and canals in Australia – an issue that is particularly important when one source is saline or otherwise polluted or where losses from surface waterways are of concern.…”

“…They also recommended that the vast volumes of data collected during such surveys are most robustly interpreted by using a series of closely spaced 1D layered earth inversions rather than 2D inversions – presumably because the latter are more susceptible to bends in the electrode array geometry associated with course changes (c.f. Snyder et al ).…”

Investigating sediments and rock structures beneath a river using underwater ERT

“…Therefore, electrical resistivity and geometry of the water column must be known for an accurate robust inversion [12,39,44,45]. The electrical resistivity and geometry of the water column is fixed in the earth model and the inversion program attempts to determine the electrical resistivity of the cells that would most accurately reproduce the observed electrical resistivity measurements [46].…”