Geophysical-Geological Exploration And Evaluation Of Offshore Sand And Gravel Deposits

Abstract

ABSTRACT Offshore sands and gravels are potentially important resources, especially near high-use urban centers, which usually lack nearby land sources and thus require transport of this low-cost, high-bulk commodity from a distance. Exploration for offshore deposits by conventional acoustic profiling and coring is difficult, however, because the high acoustic impedance of sands and gravels minimizes sound penetration, and because conventional coring is generally impossible. In the summer of 1976, our initial offshore experiments using combined resistivity and acoustic profiling at 3 test areas along the shore of western Lake Michigan, where coarse bottom material predominates, indicated that resistivity profile measurements using surface-towed Sch1umberger electrode arrays differentiate between sand and gravel and till on the one hand and sand or clay on the other. On the basis of surficial bottom grab samples and especially from profiling in the near shore off adjacent land deposits, we infer that logical offshore extensions of thick, coarse deposits are characterized by high resistivity values (~500 ?-ft.). Further, in some areas the contours of resistivity values in shallow water often agree better with logical antecedent trends roughly perpendicular to the shore than with the present surficial sediment distribution pattern, suggesting that offshore resistivity profiling is a very efficient way to, e.g., prospect for buried stream channels and to limit targets to thick, accessible deposits. INTRODUCTION Domestic demand for sand and gravel has been increasing at 5.5% annually since 1948 and is predicted to grow at a rate of 3.9-4.7% for the rest of the century (Cooper, 1970). The major consumption of sand and gravel is in urban areas. Since sand and gravel is a low-cost, high-bulk commodity, the delivered price is sensitive to the distance between source and use areas. For economic reasons, then, sources have been sought near urban zones, the centers of highest consumption. As nearby sources are, depleted or made unavailable by urban expansion inland sand and gravel has to be moved from more distant sources, the increase in price of the aggregates becoming equal to their original worth for every 32-40 km of truck transport (Cruickshank, 1975). A possible alternative nearby source for cities along coasts is offshore sand and gravel deposits. Marine-dredged aggregates make up a large part of the total production in the United Kingdom (Archer, 1973), being competitive with land sources at equivalent distances (Hess, 1971). Other countries, such as Denmark, the Netherlands and Japan, also now get 15–19% of their aggregates from the adjacent continental shelf (Cruickshank and Hess, 1975). In the United States, there has been a search for offshore sand and gravel along the Atlantic coast off New England (Schlee, 1968), the Gulf of Mexico, the Pacific coast off California, and off Hawaii (Campbell et a1., 1970). Until now, there has been no effort to assess the offshore sand and gravel resources under western Lake Michigan, even though an early reconnaissance study (Emery, 1951) showed gravelly and sandy bottoms common from the near shore to about the 100 ft. isobath. Further, sand size regolith has been more recently verified here (Lineback et a1., 1972, 1974).