Seismic surveys may also be performed in boreholes. The receivers used in borehole seismic surveys are geophones that are lowered into the borehole by a wireline. Springs or arms are used to clamp the geophones to the side of the borehole obtain good coupling. A source on the
Figure 2 depicts the difference between surface seismic acquisition and borehole seismic acquisition. Note the in the borehole seismic survey both the travel time to the geophone from the surface and the depth of the geophone are known, so a direct time to depth conversion can
be made after geometric corrections.
Figure 4 Borehole seismic vs. surface seismic
Borehole surveys can be divided into checkshots surveys and vertical seismic profiles, or VSPs. Checkshot surveys are run for the purpose of obtaining the time picks from the direct arrivals from the source, shown as the orange lines in figure 2. The intervals at which the geophones are placed in the borehole are generally 100 to 150 meters. Vertical seismic profiles are acquired for the purpose of processing the waveforms in a similar manner to surface seismic. The geophone interval in the borehole for VSP acquisition needs to be 8-30 meters to adequately process the data. Checkshot time, depth and velocity information can be extracted from VSP surveys.
Figure 3 depicts a zero offset source being used to acquire a checkshot in a straight well. The source is usually not right at the well for noise considerations and to avoid damage to the rig, but placed approximately 30 meters from the wellhead. Travel times must be corrected for the slanted path due to the source offset, a time shift caused by the depth or elevation of the source and for the elevation of the well datum relative to the seismic reference datum. The average velocity from the datum to the geophone may be calculated by dividing the depth by the corrected time. Velocities for the interval between two geophones may be calculated by dividing the difference in the depth of the geophones by the difference in corrected times.
Figure 5
Figure 4 shows a zero offset source used for a checkshot shot in a deviated well. Using this geometry there are larger corrections to be made for the slanted path that the energy takes to get to the geophones in the borehole and if there are lateral velocity variations in the earth around away from the well the vertical travel time may not be representative of the actual travel time above the geophone location. To overcome these problems the checkshot may be acquired in a walkabove geometry as shown in Figure 4 where the source is placed above the geophone in the borehole so that the travel path is vertical and representative of the travel time in the underlying layers. In practice the same source location may be used for several geophone positions to save costs, but a near vertical path is maintained. The travel times are corrected for the acquisition geometry, that is, the source depth and offset and the borehole trajectory to give a vertical time and depth.
Figure 6
The checkshot datum is the elevation at which time equals zero for the checkshot. If the checkshot times are to be compared to surface seismic data the checkshot times and the surface seismic must be adjusted to the same elevation reference. To determine the time shift to be applied to the checkshot times both the elevation of the checkshot datum relative to the common reference and the average velocity between checkshot datum and the common reference must be known since time is equal to the distance divided by the velocity.
The results of the checkshot are lists of time � depth pairs. Logs of the average interval and RMS velocities scaled in time and depth may also be delivered.