Reports
The electric log: Geophysics' contribution to ground-water prospecting and evaluation
1967, Hansen, H.J.
Information Circular 4
From the text:
The middle decades of the twentieth century have been characterized by an explosive advance in the exploration of both inner and outer space. The Surveyor and Explorer probes to the moon and inner planets, along with the Moho project to drill through the Earth's crust have demonstrated forceably to the public the technological strength available to modern man. The keystone to much of this advancement has been the development of a wide spectrum of electronically operated telemetric and remote sensing devices. These devices have extended the range of Man's sensory perception by permitting him to record environmental conditions that would be impossible to obtain by direct observation.
Although most of us will get no closer to space exploration than the morning newspaper, many of the electronic sensing devices presently available are of practical value to us. One of these is the electric well log, a geophysical tool used for prospecting and evaluating underground water supplies.
Generally speaking a well log is a verbal or graphical description of any borehole characteristic in terms of depth. The most common well log is simply a description of the cuttings obtained during the drilling of a well. Such a log is called a Formation Log. The Formation Log is compiled by a driller or geologist after he has visually inspected the borehole cuttings; it is a direct form of logging. The Electric Well Log, on the other hand, is an indirect technique that records at the surface electrical parameters that are being sensed from within the borehole itself. Electric logging is the most important of the so-called "wireline" logging techniques. Essential to all of these techniques is the lowering of an electric sonde or probe down the borehole. In the case of the electric well log the sonde is capable of recording the electrical resistivities of the subsurface formations as well as the spontaneous potentials generated in the borehole. Other "wireline" logs measure the natural radioactivity and temperature of the subsurface formations.
Electric logging methods were first introduced to meet the needs of the oil and gas industry as a technique that could accurately record and identify the characteristics of subsurface strata encountered during their drilling operations. Formation logs based simply upon the description of drill cuttings proved to be much too generalized for their detailed investigations; core drilling, on the other hand, was inordinately time consuming and expensive.
In 1927 the Schlumberger brothers successfully tested the first practical electric logging system in a French oil field. The use of electric logs quickly mushroomed throughout the world and by 1933 had been introduced to all the major oil producing districts of the United States.
Advances made in borehole technology by the oil industry are in many cases applicable to water well drilling. As would be expected, however, there is a certain amount of lag time between innovation and use. This lag in utilization is needed because the oil field techniques must first be scaled down to fit within the more limited economic framework of the water well industry. In the case of electric well logging the lag has been considerable, particularly in areas such as Maryland that are outside the mainstream of oil and its related activities. Within the last five years, however, the use of small portable electric logging units within the Coastal Plain areas of Maryland has accelerated to the point where many of the larger municipal and industrial wells are now being routinely logged.
