Ocean Engineering Applications for EIectrical Resistivity Techniques

Abstract

ABSTRACT An electrical resistively measuring system (ERMS) has been designed and built for the in-situ determination of marine sediment properties and engineering characteristics. The development of the ERMS resulted from extensive laboratory tests on the electrical properties of a wide variety of sediments (sands, silts, and clays) having porosity values from 26 to 93 percent. The laboratory data indicated that porosity predictions to within ± 2 percent were attainable by electrical resistively techniques. In addition, the sediment particle size, distribution, and structure influenced the electrical resistively measurement independent of porosity, While particle shape did not. To date, the ERMS has been used in the marine environment to predict and measure sediment type, porosity, water content, consolidation coefficients, and changes resulting from oil/gas and other pollutants. The system also had detected sediment core losses, bottom touchdown, and sample disturbance. In addition, the relatively inexpensive system could easily be adapted to temporal ocean-engineering applications such as monitoring scouring action and measuring erosion or deposition phenomena. INTRODUCTION Ocean-engineering disciplines such as underwater acoustics and submarine soil mechanics require the knowledge of in-situ properties of marine sediments. One property of special interest is porosity because of its direct relevance to the engineering characteristics of sediments. Currently, marine sediment samples are obtained from the ocean floor and transported to a laboratory for porosity determination. However, both the collection and transportation processes disturb the delicate soil matrix and may cause a change in the porosity data as subsequently determined in the laboratory. Electrical resistively measurements have long been taken by the geophysical well-logging community to predict the in-situ porosity of consolidated formations. A literature review (see Erchul) revealed that recently engineers and marine geologists have taken in-situ electrical resistively measurements of marine sediments. In most cases the instrumentation used was similar to well-logging equipment and only measured large changes. It was the author's opinion that electrical resistively could accurately predict porosity of marine sediments; however, certain equipment modifications would be required. In addition, the determination of sediment physical characteristics effects on the electrical receptivity measurements would have to be made. Laboratory tests were first conducted to determine how accurately sediment porosity could be predicted using a variety of electrical resistively techniques. Also investigated was the effect on electrical resistively measurements of the following sediment physical characteristics:type of sediment (sand, silt, clay),size of particles,shape particles,distribution of particle size,structure of particles, andmineralogy. After obtaining promising laboratory results, an electrical receptivity measuring system (ERMS) was designed, built, and tested in a marine environment to predict sediment porosity. The ERMS was also used in experiments to determine consolidation coefficients and indicate changes resulting from oil/gas and other pollutants.