Gravity Surveying in Early Geophysics. II. From Mountains to Salt Domes

Abstract

Progress in measurement of the force of gravity and its contribution to the understanding of geology, and to exploration for oil and mineral deposits, from the eighteenth to the early twentieth century is reviewed. Initially, work focused on determination of the mean density of the Earth. Pendulum observations during the trigonometric survey of India (1805-43) revealed a low-density zone beneath the Himalayas and led to development of the Pratt and Airy compensation models in 1855, followed by Dutton's concept of isostatic compensation in 1889. Use of the Eötvös torsion balance (1889) to map the gravity field over an oil-bearing structure in 1915-16 heralded economic applications. By the 1920s, it was being widely used to search for oil-bearing salt domes, coal and mineral deposits. With the introduction of the gravity meter in the 1930s, the torsion balance fell into disuse and the modern era of gravity surveying and prospecting began. With the development of progressively more sensitive instruments, the gravity meter has retained its place, becoming an essential companion to 3-D seismic surveys and, with new instrumentation, gradiometry has seen a revival as an extremely powerful exploration tool.