Gold Prospecting Methods
-Seismic Surveying

Seismic: The seismic refraction method, due to its versatility, is one of the most commonly used geophysical methods in engineering, mining, groundwater exploration and environmental site investigations. Based on favorable density contrasts that generally exist between geological materials, the refraction method is utilized to provide detailed information on the distribution and thicknesses of subsurface layers with characteristic seismic velocities. Overburden and basement rocks may be classified to some degree to discriminate for example, glacial tills from gravels or highly fractured rock from competent rock. The technique is widely used for rippability assessment of bedrock.Fig. 4.2 The Seismic method and its determinations

The Seismic investigations yield a great variety of reliable data:

• the depth of various overburden layers
• the depth to bedrock
• the soil composition and solidity
• the rock quality
• the depth to water table
• the rock structure

Field operations involve laying out a seismic cable with several geophone detectors (usually 12 or 24), at the takeout points on the cable. In some situations, such as in saturated sediments, shear wave information is more diagnostic of layer information than compressional wave. In this case a shear wave source and shear wave geophones are employed. Overwater, pressure-sensitive hydrophone receivers are substituted for the geophones. Geophone or hydrophone spacing is strongly dependent on the depth of search and the desired resolution for a given survey. A pattern of shot points is then executed within and off the ends of the cable and the seismic wave arrivals for each geophone are recorded in the seismograph. The key piece of recorded information is the time of the first arrival. This arrival is the direct wave, or more commonly, the refracted wave which occurs when seismic energy propagates along a geological interface having a sufficiently great velocity contrast. This contrast must consist of a higher velocity zone underlying a lower velocity zone, fortunately the most common geological condition.

The energy source may be sledge hammer blows in extremely shallow search surveys (less that 10 meters), a shotgun source when overburden conditions allow, or explosives where depth and/or energy attenuation is a deciding factor. The maximum depth of exploration is limited by space requirements for long cable layout and favorable shooting conditions for explosive charges. In general, a seismic cable three times the expected depth of exploration is required to ensure sufficient bedrock or basal layer arrival information to provide depths independently beneath each geophone location.

Interpretation of the seismic data involves resolving the number of velocity layers present, the velocity of each layer, and the travel time taken to travel from a given refractor up to the ground surface. This time is then multiplied by the velocity of each overburden layer to obtain the thickness of each layer at that point. The analysis of the refraction data is assisted by the use of an integrated suite of programs.

In some circumstances companion surveys may be carried out to provide correlative information. Transient electromagnetic soundings, resistivity soundings, or multielectrode resistivity surveys provide a means of assessing additional layering information. Frequently, the marine seismic refraction method is a companion survey to marine seismic reflection profiling surveys.

The seismic refraction method is generally utilized for determining depth to and competency of the bedrock surface, overburden classification, geological structure, and earthquake risk assessment. On land and on the ocean bottom, shear wave refraction is used to determine layering and layer elastic modulo.

Electrical resistivity methods have been traditionally used for subsurface layering delineation based on distinct resistivity contrasts between diverse geological materials. Generally utilized to obtain resistivity soundings, the method is also used in the profiling mode to provide more rapid coverage of large survey areas.

Overwater acoustic profiling is carried out to determine sediment layering and depths to bedrock in the marine or fresh water lake and river environments. The tool is usually accompanied by high frequency fathometer and side-scan sonar. The side-scan sonar is used to scan to the sides of a traverse to determine bottom conditions and locate bottom or shallow sub-bottom hazards. Positioning is generally carried out with standard GPS technology.