Seismic and resistivity exploration of soils and shallow bedrock in forestry research and management

Abstract

Seismic waves and electrical current can be used to provide a great deal of information about soil and rock layers, rapidly and cheaply, without digging or blasting. While these methods are by no means new, their potential has not been realized in forestry. However, instrumentation and techniques have now advanced to the point where interpretations can usually be made with confidence. Obvious applications are in road alignment, dam location, etc. The methods could be used in forest research to locate field experiments in areas of sub-surface homogeneity; this also would offer great potential savings in terms of lowering experimental error and thereby raising efficiency. Seismic refraction methodEarthquakes, both naturally and artificially produced, e.g., by a charge of dynamite or the dropping of a hammer, send waves through the ground in all directions from the point of origin. Seismology is the science and studv of these waves which travel at velocities that debend largely on the density of the material traversed. Waves may travel to a point both directly through the medium, or indirectly by reflection from, or refraction through, an interface. The seismic refraction method is based on the measurement of the rate of advance of man-initiated wave fronts.Seismology has for some decades found applications in deep geological exploration for mining and oil and other engineering. For such work, waves are generated by exploding a charge of dynamite. Within the last few years, advances in electronic engineering have made possible the development of sensitive, lightweight, portable instruments such as Soiltes@' Inc.'s R150, MD-3, and MD-5 Portable Refraction Seismographs and Huntec Ltd.'s FS-3 Portable Facsimile Seismograph. These instruments are able to time the arrival of wave fronts generated by the impact of a hammer on the ground, and they can be used to explore shallow soil and rock layers to depths of perhaps 100 ft. These are the layers that are of prime importance t o the forester, be he manager or researcher. The instruments measure the time taken for a wave (produced by pounding either the ground itself or a metal plate on the ground with a hammer) to travel from the point of impact to a geophone detector placed a known distance away. A t the instant of impact, a shock switch on the hammer activates the timer which runs until the wave front reaches the geophone.The MD-3 seismograph has direct digital readout and automatic reset. The MD-5, a newly developed seismograph, incorporates such features as high brightness, 0.75-in. numerical readout. The R150 uses a cathode oscilloscope tube to display the actual wave received by the geophone. For some purposes, the nature of the wave itself is a valuable interpretative aid. For instance, seismic interference is more easily identified and interpreted with the wave-display instrument than with the digital readout. The FS-3 instrument is able to eliminate unwanted noise signals originating from sources in line with its dual geophones; and a permanent record on dry pap...