Water-Borne Resistivityllnduced Polarization Survey in Prudhoe Bay
Authors: Scott W.J. , Laing J.S. , Botha W.J. 
Published: October 2008

ABSTRACT
During August 1982, a marine resistivity/induced polarization (IP) survey was undertaken in Prudhoe Bay, Alaska. Multi-electrode arrays with inter-electrode spacings of 10 metres and 25 metres were used to survey a total of 34 line kilometers. Apparent resistivity values were considerably higher over known granular materials. No strong IP responses were observed. The use of marine resistivity measurements in conjunction with high-resolution sub-bottom profiling can improve the chances of finding granular materials in shallow offshore waters.

INTRODUCTION
During late August 1982, a marine resistivity/ induced polarization (IP) survey was undertaken in Prudhoe Bay, Alaska. The purpose of the survey was to determine whether the system could be used to detect and map gravel deposits in the Bay.

TECHNICAL APPROACH
From measurements of resistivity made on land it is clear that the resistivity of sub-surface materials increases with increasing grain size. Thus coarse-grained materials such as sand and gravel have higher resistivities than fine-grained silts and clays. This resistivity-grain size dependence remains true even when the materials are saturated with saline water.
Sub-bottom materials tend to have higher resistivities when frozen (i.e. ice-bonded) than when not frozen. The presence of permafrost in the subbottom will be observed as higher resistivities. A frozen clay could in this manner give rise to apparent resistivities comparable to those associated with unfrozen coarse-grained materials. However there is evidence (Olhoeft 1975) that frozen clays exhibit an induced polarization response which is not present in unfrozen coarsegrained materials.
The IP effect can be observed as a small transient voltage which exists in a polarizable medium after interruption of electric current flow. This transient is generally two orders of magnitude lower than the voltage drop in the medium during current flow. It has time constants in the 0.1 to 10 second range. (IP effects also arise in the presence of metallic sulphides; IP measurements are frequently used in prospecting for metals).
The measurement of apparent resistivity and IP effects can be expected to provide sufficient information to detect and map gravel distribution in the sub-bottom. High resistivities with no IP response would imply the presence of gravels, while high resistivities and an IP response would suggest the presence of frozen fine-grained sediments This situation is analogous to that observed on land, where measurement of electrical response is a widely-accepted means of mapping granular deposits and permafrost distribution.

RESISTIVITY AND INDUCED POLARIZATION MEASUREMENT
The resistivity/IP measurement system used in this survey has been developed by Hardy Associates (1978) Ltd. over the past three years. Figure 1 is a schematic illustration of the system.
The system measures the electrical resistivity of sub-bottom materials by injecting electric current into the water from a pair of electrodes and measuring the resulting voltages between successive pairs of electrodes equally spaced at increasing distances from the current electrodes. The electric current is supplied by a Huntec (70) Ltd. M4 IP transmitter, capable of producing 20 amperes at power levels up to 7.5 KVA. The current waveform is an interrupted square wave with a period of 4 seconds.