A new resistivity meter, featuring fully automatic measurement and a built in analysis program

Abstract

Vertical electrical sounding is applied to a wide variety of objectives, including mineral resources, thermal energy, and water resources. Thus, there is a great need for electrical exploration instruments that minimise the effects of noise from stray currents or electromagnetic coupling, as well as from anomalies such as local variations in shallow underground soil or changes in topography. In addition, there is a demand for more rapid measurement capability, cost reduction and instruments that can conduct in-situ measurement automatically. Methods that have been proposed for improvement of S/N ratio include the Offset Wenner method, proposed by Barker (1981), and the synthetic potential method using the equidistant spacing of 4 electrodes, proposed by Sakayama and Shima (1985). In the latter case, they check noise contribution and eliminate the effects from local anomalies. The Offset Wenner method and digital stacking resistivity meter can be applied to the problem of achieving more rapid measurement and cost reduction. In regard to analysis, the appearance of the linear filter method has simplified forward calculation. The development of a stable algorithm using a non-linear type method of least squares has made the use of mini-computers, to conduct inversion for vertical exploration, commonplace. With the background of this kind of technological advance, we have developed a new generation all-in-one type electrical resistivity apparatus, the McOHM II. The McOHM II combines in a single package an electrode switchover scanner, a resistivity meter, a vertical exploration analyzer and a printer/RS-232C output section. The operation of each section is controlled by a microprocessor. Measurement is automatic, with measurement error monitored by the microprocessor, always maintaining optimal settings. The McOHM II does not require a mini-computer or personal computer; its ROM base determines resistivity structure from measurement values automatically. Measurement data and analysis results are stored in the memory, and can be output on 80 mm wide paper from the printer. With the features described above, the McOHM II merits the title 'new generation electrical exploration apparatus'. The paper summarises specifications and features of the McOHM II, and gives examples of its use in field surveys.