Down-hole and Reference Geophones

The BSU sign convention for geophones is similar to that for sources. An arrow or unit vector is associated with each geophone element. Ground motion in the direction of the arrow will generate a negative voltage at the geophone, and be recorded as a negative number on the seismic trace. For a typical moving coil “velocity” phone, this voltage is proportional to particle velocity of the soil, as sensed by the geophone at the point where it is clamped to the soil. Figure 11 shows a plan view of the typical survey for which BSU was written.

Figure 11: Plan view of a typical survey. Coordinate system for geophone components and impact forces.
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There are two 3-component geophones. One is fixed at the surface (or buried very shallow). This reference phone provides a way to monitor variations in the source waveform and recorder triggering. In Figure 11, the horizontal, R-component of the reference phone is designated (0,90), and the T-component (270,90). The vertical component arrow (not shown) is (0,0), pointing to zenith. The azimuth for the vertical component is meaningless, and so is set to zero for simplicity. The down-hole phone is free to spin on the way down, and so its orientation must be determined for each down-hole station. This is the job of the Principal Component Analysis (PCA), and will demonstrate the use of the program, bhod, named for the hodogram in the horizontal plane. The use of the labels R and T on horizontal components is purely arbitrary in down-hole surveys. You will note that in Figure 11, the chirality is not consistent (left handed vs right handed). Your geophones may be quite different, but the sign convention is what is important, and this can be determined by a tap test, or a shallow level where the tool orientation can be observed directly by sight. A very important observation at the end of each survey is to note and record the bow spring azimuth as the tool exits the hole. On the GeoStuff tool, the spring is aligned with the R-component. Knowing this direction is essential to establishing a guide vector for the PCA analysis which has an inherent 180 degree ambiguity. The vertical down-hole component points to zenith in the GeoStuff tool. Not all tools are constructed this way, so you need to check your tool to determine its actual orientation. The down-hole vertical component is designated (0,0).

Each seismic trace will have header values for the geophone and source polarization (unit vectors in spherical coordinates). These are stored as integers in the BSEGY header. Aside from looking at the include files, the header definitions can also be reviewed with the man pages. For Fortran definitions, type:


man bsegy


You will find integer*2 header values for shot and geophone polarizations (shtazi, shtver, geoaz, and geover). For C-language definitions, type:


man c_bsegy


Thus, in C you will find the header structure elements hd.geoazi and hd.geover contain the azimuth and vertical angles of the geophone. The corresponding shot polarization is stored in hd.shtazi and hd.shtver.