BHOD

Hodogram analysis by Principal Component Analysis (PCA) can determine the orientation of the horizontal components of a 3-C geophone in a bore hole. Program BHOD does this analysis and outputs a file, bhod.lst that has rows of 3 numbers, (seq., R-azimuth, T-azimuth). The sequence number corresponds to the *.seg files. File, bhod.lst, is then used by BTOR 12.2.2 to update headers. Program BROT 12.2.4 is then used to rotate the data to a desired orientation.

In the case of a horizontal impulse source, two opposite polarities will be struck for each geophone depth. Depending on which source blow azimuth is first, the bhod.lst sequence number will either be the first or second blow, and the result of PCA will be applied to both source efforts at the depth being analyzed. In a typical survey there will be twice as many *.seg files as depth stations occupied. One surveys from the bottom to the top of the hole, and should make an IMPORTANT observation of the tool orientation at the surface to resolve the $180^0$ ambiguity. Helper scripts are generated by GENBHOD 10.1.9.

In the case of a vertical impulse source, experimental helper scripts are generated by program GENBHODV 10.1.10. In this case, the procedure is designed to observe the large amplitude hodogram motion (which may be a Rayleigh wave). Rayleigh waves are a mix of P-SV motion. The P-motion is horizontal and may provide orientation information. Your mileage will vary depending how deep the Rayleigh waves motion penetrates.

The command line arguments to BHOD are:

 bhod   infile chR chT ipct saz azctl tsw1
  
 infile =input file name 
 chR    = channel with R-component (int) 
 chT    = channel with T-component (int) 
 ipct   = percent of max amplitude to include (int) 
 saz    = source azimuth (ie E-W, then 90 deg) 
 azctl  = desired direction for R- (bowspring) 
        (azctl resolves 180 deg ambiguity PCA) 
 tsw1   = switch to set T-comp relative to R-comp 
        T-comp Azimuth= R-Comp + tsw1
        Typically, tsw1= +90.(downhole) or -90.(ref)

EXAMPLE:

 bhod h002001.seg  2  3  50  90.0  315.0  +90.0

bhod.lst file:

  01002  258.6  348.6

Figure 61: BHOD: plot produced showing PCA results for a geophone at about 20 meters depth.