Geophysics as an Engineering Tool
Authors: Lindseth Roy O. 
Published: December 2007

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Engineers, geologists, and petrophysicists often practice their professions in isolation, considering each activity a separate science. This is not always intentional, but limited time and resources accentuate the effect by narrowing the scope of activity and investigation. The professional engineer is inquisitive by nature and probes professional engineer is inquisitive by nature and probes other fields, but in geophysics, for example, the rate of change is so rapid that even specialists cannot maintain reasonable knowledge of the state of the art.

Yet, knowledge of new developments in other fields is fundamental because a technique effective in one field often can be adapted for another. For instance, one successful method of seismic exploration was adapted from radar techniques. Petroleum engineers and exploration geophysicists draw freely from each other's disciplines.

One recent technological transfer, not yet widely available, is the adaptation of the geophysical gravimeter to borehole logging. A gravimeter measures changes in density, so it can be used to locate subsurface structures or salt domes.

A gravimeter has been designed to fit into a logging sonde and to withstand extreme pressure and temperature encountered at depth. In the sonde, the apparatus is oriented to measure lateral expression of gravity, and thus can measure the relative density of rock surrounding the borehole. Casing in the hole, which precludes valid readings on many types of logs, merely applies a small constant shift to measurements, making the gravimeter equally effective in cased or uncased holes.

Porous formations, particularly when filled with gas, are usually less dense than surrounding rock and cause reduced gravity readings. Therefore, the device is well adapted for detecting porosity, even behind casing.

The device cannot operate continuously because acceleration and gravity are related functionally; it must be stationed at each level to be measured. Hence, conventional logging is faster, but for many applications the device can be targeted for a relatively narrow zone of interest.

Another new geophysical development is related to digital computing. Seismic surveys are interpreted in terms of structure. Although potential reservoirs may be located this way, little important information about the nature of the rock (lithology, porosity, and permeability) can be gathered by surface-based techniques without first drilling a borehole.

Amplitude of a signal returning to the surface from a given reflector, when compensated for transmission losses, is a direct function of the velocity change across the boundary it maps. Until recently, little attention was paid to the strength of the returning reflection because it paid to the strength of the returning reflection because it could not be measured reliably, but now it is possible to measure it with reasonable precision.

The first application of this new information was rather simple. Average velocity through porous rock filled with liquid will decrease if the fluid is replaced by gas. If a continuous reservoir bed, normally containing water (or oil), crosses a trap where gas replaces the fluid in the pore space, the sudden change in velocity through the reservoir rock results in a changed velocity contrast across the boundary between the reservoir rock and adjoining formation. This, in turn, changes the amplitude of the reflection, often by a readily measured amount. The resulting amplitude change, termed a bright spot, is used to indicate gas directly.

Unfortunately, other changes in lithology produce similar effects. For example, a coal seam produces a response similar to gas-filled porosity. These failures do not discredit the method, but merely show that at best geophysical surveys indicate subsurface conditions imperfectly.

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