CROSSHOLE SEISMIC TESTING

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Crosshole (or “crosswell”) seismic measures the velocity of seismic waves between boreholes. There are two types of crosshole approaches. The conventional approach involves lowering a 3-component borehole geophone down one hole while lowering a source down an adjacent hole(s), firing the source at some prescribed depth interval. The source and geophone are always at the same elevation, and the energy from each shot is measured at a single depth in each receiver hole. The traveltimes are then converted to velocities by dividing them into the distance between the holes.

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This method can provide very detailed seismic p- and s-wave velocity information between closely-spaced boreholes.​

 

COMMON APPLICATIONS

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• Bridge/dam foundation analysis

• Insitu materials testing

• Soil and rock mechanics

• Earthquake engineering

• Liquefaction analysis

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CONSIDERATIONS

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• One of the most common mistakes made by inexperienced practitioners of conventional crosshole seismic is mistaking refracted energy for direct energy. Depending on layer thicknesses, distances between holes, and velocity contrasts, the first-arrival energy is quite often refracted rather that direct. Refracted travel times must be corrected prior to computing velocities.

• While downhole sparkers are available and generate good p-wave energy, shear wave velocity is difficult to measure in crosshole seismic. Commercial availability of shear wave sources is limited, and these sources are rather difficult to use.

• It is difficult to achieve “perfectly” vertical and straight boreholes. There is always some deviation in both parameters. And since crosshole is often done in high-velocity material and closely spaced holes, assuming straight and vertical holes can lead to significant errors. A borehole deviation survey is therefore imperative.

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BENEFITS/LIMITATIONS

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• Crosshole seismic, unlike seismic refraction, does not require that velocities increase with depth.

• If a borehole hydrophone or hydrophone array is used, the receiver hole must be filled with water.

• Similarly, a borehole sparker requires that the hole be filled with water.

• Refraction effects, discussed above, put an upper limit on the distance between the holes and the number of individual layers that can be resolved.  However, in general, resolution is better than that of seismic refraction.

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DELIVERABLES

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The final product of a crosshole survey is a velocity model such as that shown below: