NexTerra - Surveys

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Surveys - Why Geophysics?

Rapidly growing population and modern life style adds up to the ever-increasing demand for higher energy and other economic mineral resources. Most of the surface and near surface minerals have already been exhausted and we are left with no other option but to explore deeper into the mother earth. As only a tiny fraction of rocks that make up the Earth can be studied directly, geophysical exploration techniques are essential for the economic exploitation of mineral, water and hydrocarbon resources. The same techniques play key roles in modern archaeology and civil engineering. Whilst monitoring and modelling the storage and movement of fluids in the crust and sub-surface reservoirs is as important for waste disposal and maintenance of aquifer quality as for oil extraction and carbon sequestration.

The aim of geophysics is chiefly to decipher what is inside the earth based on scientific measurements near its surface. Geophysics is the study of the earth by quantitative physical methods, especially by seismic reflection and refraction, geodesy, gravity, magnetic, electrical, electromagnetic, and radioactivity methods i.e. the use of observations of physical phenomena at the surface to infer the deeper structures of the Earth.

The most effective employment of geophysical methods is during the early stages of an exploration project, rather than as a last resort when all else has failed. A well-designed geophysical program at the initial stages of a project can provide critical information to help locate unexpected hazards, strategically place boreholes, and control contractor bids. At most sites, geophysics is not a substitute for drilling, but it can be used to strategically place boreholes, and optimize the information provided by the ground sampling. Soil samples and boreholes provide detailed information of subsurface conditions on a small scale at specific locations. Conversely, geophysics provides continuity of general subsurface information between sample points, reducing the risk of encountering surprises that can result in cost overruns.

Limitations

Often overlooked when proposing geophysical investigations are their inherent limitations. Geophysics relies on contrasts in physical properties between subsurface layers, or buried objects, in order to image targets. The geophysical methods employed must be sensitive to the property contrasts that are relevant to the site. All too often, inappropriate methods or acquisition parameters are used in environments where a careful analysis of the survey objectives and greater experience with a wider array of geophysical techniques would have suggested alternative methods or rejected the use of geophysics entirely. Even when the appropriate techniques are used, the scale of the property contrast must be sufficient for target discrimination. Of additional consideration is the depth of investigation and the resolution required. Various geophysical methods have different optimal penetrations, and can provide substantially different subsurface detail. In general, the greater the penetration depth required, the less the detail of the information obtained.

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