GEOPHYSICAL METHODS

Crosshole/ downhole/ uphole Surveys

Theory:
Crosshole testing takes advantage of generating and recording (seismic) body waves, both the P- and S‑waves, at selected depth intervals where the source and receiver(s) are maintained at equal elevations for each measurement.  Figure 1 illustrates a general field setup for the crosshole seismic test method.  Using source-receiver systems with preferential orientations in tandem (i.e., axial orientations, which complement the generated and received wave type/signal) allows maximum efficiency for measurement of in situ P- or S-wave velocity depending on the axial orientation.  Due to the different particle motions along the seismic ray path, it is crucial to use optimal source-receiver systems in order to best record crosshole P- or S-waves (Hoar, 1982). More...

Data Acquisition:
Recording instruments used in crosshole testing vary considerably, but there are no standard requirements other than exact synchronization of the source pulse and instrument trigger for each recording. Crosshole measurements rely considerably on the premise that the trigger time is precisely known as well as recorded. The recorded trigger signal from zero-time geophones or accelerometers mounted on the downhole impact hammer allows accurate timing for the first arrival at each drill hole. This becomes uniquely critical when only two drill holes are used (i.e., source and one receiver) because there is no capability of using interval travel times; in this case, the velocity is simply determined through distance traveled divided by direct travel time. Utilizing digital recording equipment affords the operator the ability to store the data on magnetic media for analysis at a later date; but more importantly, digital data can be filtered, smoothed, and time-shifted during analysis. More...