This technique measures vertical changes in seismic velocity by placing a source at the top of a borehole and measuring travel-times at multiple intervals in the borehole, usually with a 3-component geophone. Common applications include:
- Bridge/dam foundation analysis
- Insitu materials testing
- Soil and rock mechanics
- Earthquake engineering
- Liquefaction analysis
- It is important that the hole is cased and well-grouted to ensure good seismic coupling between the geophone and the surrounding soil and rock. See the crossholeASTM for good grouting procedure; it applies to both downhole and crosshole.
- Although similar to crosshole in that a borehole is used, in this case the source stays on the surface. This means that the raypaths are largely redundant from one geophone depth to the next, and you are looking more at the difference in travel time with depth – you are looking at the slope of the travel time curve, rather than the absolute travel time for each shot, much like seismic refraction. This, at least theoretically, means that downhole is a lower-resolution approach than crosshole. On the other hand, the data analysis is relatively straightforward compared to crosshole, and much less prone to error.
- Unlike refraction, downhole looks at direct raypaths, and as a result does not require increasing velocity with depth. If there is concern about a near-surface velocity inversion, a downhole survey can shed light on this and help constrain the results of a refraction survey.
- The results of a downhole seismic survey generally include traveltime plots and velocity graphs for both p- and shear-waves, as illustrated below: