2.1 HISTORICAL BACKGROUND AND DEVELOPMENT Geophysical exploration was probably born in the early 1920s. This was after the successful development of electrical prospecting methods by the brothers Conrad and Marcel Schlumberger in France, and the seismic refraction method in the newly discovered oil fields of the mid-south USA. During the following decade experience was gained using these methods and new techniques were developed, such as seismic reflection, gravity, magnetic and electromagnetic surveying, borehole logging and the use of seismic methods at sea. The exigencies of world War II led to the adaptation of many geophysical methods for use in detecting mines, submarines and enemy fire positions. This encouraged basic research, which led to tremendous advances in electronics, signal processing concepts, and computer methods. Consequently, during the following four decades, the geophysical exploration industry in the non-communist world grew rapidly to having an estimated annual turnover of £3 billion. The bulk of this effort was concerned with the petroleum and mining industries, but there was also a steady growth in the application of various geophysical techniques to civil engineering and groundwater studies. From early applications of the seismic refraction method to the determination of depth-to-bedrock in the 1930s, the practice of engineering geophysics has expanded to encompass a wider range of techniques, applied to more types of problems, than any other branch of geophysics. To a large extent, the techniques and equipment of engineering geophysics evolved from the other sectors. The main differences relate to the relatively shallow depths of investigation