Marine resistivity is a valuable non-invasive geophysical technique which has been used in various geotechnical, geo-environmental and coastal/marine studies in order to preliminarily investigate areas of the coastline/seabed which show signs of scientific interest. The majority of these studies have examined the impact of seawater ingress into coastal groundwater aquifer supplies (especially in Karsitic environments) and have mapped the spatial and temporal distribution of freshwater discharge into the ocean. In addition to this, resistivity can be used to examine the influence of faulting, porosity, heterogeneity and void ratio of the seabed sediments, along with the tidal influence, as these act as pathways for saline intrusion.
Coastal areas are commonly evaluated using schematic geotechnical models derived from numerical modelling and borehole records. However, more reliable information is needed in the form of 2D resistivity inversion models, which allows the user to evaluate and mitigate areas of risk i.e. areas which contain highly fractured sedimentary bedrock, saturated marine clays (strong bands of clay with high plasticity index, moisture content, salt content and void ratio) and areas affected by permafrost melt. In polar regions the impact of permafrost melt may lead to areas of seabed instability as the freshwater melt will seep through the sediments/fractures, initiating collapse.
In terms of deployment, the Syscal Pro Deep Marine system can be deployed as a floating array or as a submerged array. The floating array allows the user to image not only the resistivities of the seabed but helps map the salinity and resistivity changes within the water column. The system uses a reciprocal Wenner array and involves floating 13 graphite electrodes (11 potential and two current electrodes) at 5 meters spacing, towed behind a boat travelling at 3-4 km/hr. This is a typical survey layout for a salt water environment which has a water thickness layer between 1 and 4 meters (Fig. 1), additionally a higher current output is required in sea water, in order to produce measurable potential.
The deep-water towing method (Fig.2.) involves submerging and deploying a 10 channel 13-graphite electrode array (11 potential and two current electrodes) 4 meters above the seabed, using a 25m deck lead and floats which are separated by >70m of rope. An ideal method which can help negate the effect of the water column ensuring greater depth penetration and imaging of the seabed.
When working in freshwater environments (i.e. river, estuary, reservoirs etc) it is recommended that a Syscal Pro (not deep marine) is used.