Seismic Source (SH- and P-wave)

BSU can handle a variety of seismic sources for down-hole engineering surveys. A vertical impact hammer source is used when the interest is primarily in recording P-waves on the vertical down-hole component. The hammer may be instrumented with a load cell (in which case an extra, 7th channel of data are recorded). Or the hammer can be without any instrumentation (other than contact closure for triggering). A horizontal hammer serves when the primary interest is in recording SH-waves on the horizontal down-hole components. In this example, my 135 degree inclined hammer source is used. This source delivers both horizontal and vertical motion to the ground. The source is nailed to the soil. The source is shown in Figure 10.

Figure 10: Source generates both horizontal and vertical motion
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A sign convention for hammer sources has been adopted in BSU. One mentally associates an arrow with the blow. The arrow points in the direction of the blow, and can also be thought of as a unit vector. Thus, in Figure 10, if we assume that East is on the right, the hammer blow would be represented by the spherical coordinate designation (270,135)=(azimuth, angle from the vertical). Vertical angles are measured from zenith=0 degrees, to nadir=180 degrees. Horizontal, azimuthal angles, are measured from North=0 degrees, increasing clockwise (when viewed from above), making West=270 degrees. The BSU header stores both the horizontal and vertical angle for the source.

Data collection with this source involves two source polarizations for each sampled subsurface geophone station. In a typical survey, the geophone is lowered to the bottom of the hole, clamped with a mechanical bow spring clamp (I use a GeoStuff BHG-2 down-hole geophone (4) ), and then dragged up the hole to occupy stations at a 0.25 meter interval. As the geophone remains fixed at a station, two separate seismic recordings are stored. One is for a source polarization of (270,135), and the other for a source polarization of (90,135). Subtraction of the records enhances SH-wave motion at the expense of Rayleigh and P-waves (as seen on horizontal components). Summing the two records has the opposite effect. For a more detailed discussion on wave field enhancement, see Michaels (12).