Crosshole And Uphole Tomograph

Electrical Image of a TunnelIn some settings, tomographic (tomo=slice, graph=picture) measurements between boreholes or wells (crosshole) or between boreholes and the ground surface (uphole) can provide subsurface images superior to those based only on surface measurements. These settings include targets that are very small relative to their depth, targets that lie beneath buildings or other structures, or targets beneath sites with shallow or surficial interference or “noise”.

Deployment of a Downhole Hydrophone CableTomography typically involves sending signals between a transmitter in one borehole and a receiver in another borehole.  The signal can be a seismic/acoustic pulse, an electrical current, or an electromagnetic field or radar pulse. The advantage of tomography over surface geophysics is that signal paths are one-way and much shorter – i.e. from one borehole to another rather than down from the surface and back up again. The shorter signal paths mean that each signal samples a smaller volume of subsurface material (creating higher resolution images), and suffers lesser signal loss (creating the ability to image more subtle targets).

Three-Sided Seismic Image of a Clay LayerEnviroscan has employed crosshole seismic, electrical, and radar tomography to produce high-resolution images of limestone cavities beneath bulk oil storage tanks, bridge abutment and retaining wall cross-sections, coal mine workings, military tunnels, buried caches of various materials, the top-of-bedrock beneath large industrial facilities, and non-aqueous phase liquids (NAPL). The disadvantage of tomography is that boreholes are required, and plan-view or map-view data coverage is constrained by the locations of holes.  However, with enough boreholes at strategic locations and/or with supplementary surface geophysical measurements (where possible), geophysical tomography can provide three-dimensional mapping rather than just two-dimensional images or cross-sections.Seismic Image of a Highway Retaining Wall Cross-Section

As with any geophysical survey, the exact choice of a tomographic technique (e.g. seismic versus electrical or electromagnetic) is dictated by the physical characteristics of the target and its surroundings, as well as any potential sources of interference or noiseCrosshole Survey at an Active Service StationCrosshole Survey at a Tank Farm.

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