Surface seismic techniques used in gold exploration are restricted to seismic refraction and seismic reflection methods. Probably, the first one is the most employed. The equipment employed for both techniques is very similar and assure the travel time of acoustic waves propagating through the subsurface. In the seismic refraction method, the travel time of waves refracted along an acoustic interface is measured. In the other technique, the travel time of a wave which reflects off an interface is measured.
The information to be obtained is dependent on the acoustic properties of the subsurface material. Specifically, their properties can identify various geological materials. In this way, the interpretation of seismic indicates changes in lithology or stratigraphy, geologic structures and water saturation zones. These techniques are commonly employed to know the depth and structure of geologic and hydrological areas.
The seismic refraction technique is a geophysical method widely used to explore the ground. Basically, seismic waves travel outward from a source and reach a detector. The detector first senses the waves that went directly to it along the ground surface, and then it senses waves that went downward, were refracted at a deep layer, then left the deep layer and came back to the surface. Due to the waves move faster in the deep layer, they take the surface waves. At a certain distance exists a crossover point, the refracted waves reach the detector first. With this initial information, a few assumptions can be considered about the place and we can know the thickness of the surface layers and its possible composition.
The information obtained can be used to make a map on bedrock topography, determine the depth of gravel or sand, delineate perched water tables, detect subsurface caverns, identify shallow faults and fracture zones, and detect large boulders.
Seismograph received data on waves travelling through different rock types
Seismic refraction explorations are based on the time required for a seismic wave to travel from a source to a receiving point. Basically, a sound can be used for the seismic source and twelve or more vertical geophones are used for the receiving points. The selection of the seismic source depends on the seismic line, the resolution required, and the environmental properties. A signal enhancement seismograph records signals from the geophones. By analyzing the arrival time of the seismic wave as a function of distance from the seismic source, the seismic velocities of the underlying soil/rock units and the depth to geologic contacts can be determined. The geophone spacing and the distance between the seismic source and the first geophone are designed to obtain the needed penetration and resolution. The method is usually employed to areas where seismic velocity increases or is constant with depth.
The seismic data are studied by plotting the arrival time of the wave at each geophone versus the distance from the seismic source to the geophone. These charts are commonly known as travel-time plots. The data have to be fitted with straight-line segments. Each line segment corresponds to a different stratus or layer. The reciprocal of the slope of the line is the apparent wave velocity of the layer. Current state-of-the-art analyses use forward and inverse modeling and ray tracing that seek to minimize discrepancies between field measured arrival times and corresponding times traced through the velocity model.
The seismic velocity of a geologic stratus can be known by the refraction method and a relative estimate of the depth to different acoustic interfaces. Seismic refraction surveys are very useful to obtain information on depth at different locations. Refraction surveys are useful in buried valley areas to map the depth to bedrock thickness of overburden. The information obtained can be related to various physical properties of the bedrock; rock types have specific ranges of velocities. For example, dolomites and granites have different seismic velocities. A key aspect of this method is the line length to me measured. It is recommended that the distance from the seismic source to the geophone station or reception point have to be three or more times the desired depth of exploration.