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.
This method can provide very detailed seismic p- and s-wave velocity information between closely-spaced boreholes.
Common applications (in order of relative usefulness)
- Bridge/dam foundation analysis
- Insitu materials testing
- Soil and rock mechanics
- Earthquake engineering
- Liquefaction analysis
- 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 signification errors. A borehole deviation survey is therefore imperative.
- Seismic refraction requires that velocities increase with depth. A lower velocity layer beneath a higher velocity layer will not be detected by seismic refraction, and will lead to errors in depth calculations. Fortunately, this is a fairly uncommon occurrence in the shallow subsurface.
- The seismic source employed must match the desired depth of penetration. For hammer and plate work, the maximum depth you can expect to explore to is about 15-20m; however, this can vary significantly depending on geology, surface conditions, cultural noise, and the person swinging the hammer.
- Refraction is a relatively broad-brush technique – it looks at gross velocity differences, and you should not expect to be able to map more than 3-4 individual velocity layers.
- Cultural noise can be a problem – it is more difficult to conduct a seismic survey in an urban environment than in a rural one. Surveying along busy roadways should be avoided when possible. Shooting at night is sometimes necessary in order to achieve acceptable signal-to-noise ratio in busy areas.
- The final product of a crosshole survey is a velocity model such as that shown below:
- The ASTM for conventional crosshole seismic testing can be downloadedhere.
CROSSHOLE SEISMIC TOMOGRAPHY
With the advent of fast computers, seismic tomography has become popular. Whereas in conventional crosswell surveying there is only a single raypath considered for each shot, in tomography multiple raypaths are measured for each shot. This is accomplished by using a multiple sensor receiver array (usually a hydrophone array like the Geometrics DHA-7).
The resulting tomogram not only delineates vertical, but also horizontal variations in velocity. This is the main advantage of crosshole tomography over convention crosshole seismic.
Most often, a downhole sparker is used as a source in crosshole tomographic surveys, and shear wave velocities are not typically measured.