Near Field BNFD

The BNFD program computes the near field seismic radiation for a homogeneous whole space (Aki & Richards, 1980) (Eq 4.23,Vol. I). While not likely to correspond to any field recording, it aids in understanding the transition from near to far field without the complexity of any boundaries being present. Command line parameters follow:

 bnfd  infile xforce vp vs den alpha fc icomp ifield
 
     infile   =  input file name (sets geometry, tmax, ntraces)
     xforce   =  point force direction 
             1=  in positive x-axis direction 
             2=  in positive y-axis direction 
             3=  in positive (down) z-axis direction 
 
     vp       =  p-wave velocity  
     vs       =  s-wave velocity  
     den      =  mass density     
 
     alpha    =  exponential decay factor (pos) for wavelet 
     fc       =  center frequency of wavelet Hz
                 (for example, try 50 for alpha and fc) 
 
     icomp    =  component of motion to output 
             1=  radial 
             2=  transverse 
             3=  vertical 
     ifield   =  fields to include  
                  (SPN) binary coded 
             0=  wavelet only 
             1=  near field only 
             2=  far p-wave only 
             4=  far s-wave only 
             3=  near and far p-wave only 
             5=  near and far s-wave only 
             6=  far p- and  s-wave only 
             7=  ALL: far S, far P, Near Field

Abbreviation SPN: S-wave, P-wave, and Near-field. Thus, (SPN)=(111)=7=all far S, far P, and N. For far-field P-wave only, (SPN)=(010)=2.

In the following example, all motions (ifield=7) are computed on the vertical and radial components. The headers are copied from an actual data set, c008.seg, to set number of samples, sample interval, and geometry. The commands are:

 bnfd c008.seg 3 800. 200. 1800. 50 50 1 7 
 
 bnfd c008.seg 3 800. 200. 1800. 50 50 3 7

The first is for the radial (icomp=1) motion, the second is for the vertical (icomp=3) motion. Figure 48 shows motion in the vertical and radial directions. The template file, c008.seg, provides header data generating offset for each trace. While there are different definitions of near field in the literature, the bandwidth of the propagating wavelet (which sets the wavelet duration) and the difference between P- and S-wave velocity play the major role. The plots have been trace equalized using program BEQU to compensate for the dynamic variation in amplitude with offset.

Figure 48: BNFD: Computing all fields (S-wave, P-wave, Near-field) The geometry is taken from a template file, c008.seg, and spans offsets from 7 to 100 meters. As offset increases, the far field P- and S-wave motion waxes as the near field wanes.