bref - BSU program sets up matrices for inversion of delay times, refraction method
bref [ -h | line# | rmin | rmax | irefdir | irecip | infile1 | infile2 . . . ]
Basic Seismic Utilities (BSU) reads headers of seismic profiles to extract first arrival times. Matrices and vectors are constructed as simple text files to be read by Scilab procedures which do the actual inversion and plotting of results. One can simultaneously invert multiple common shot or common receiver gathers for delay times (if offset selection is restricted to refracted head waves), or alternatively for the overburden velocity (if offset selection is restricted for direct arrivals). First arrival picks must have already been made, and the picks added to the headers. Source and receiver headers must also be present. This program only sets up the matrices and vectors. This permits the user to employ an editor and add additional constraint equations before the actual inversion in Scilab. If additional equations are added, one will manually edit both the Gxxxx and Dxxxx matrices (to maintain a conformable system, Gm=d (m=vector of delay times and refractor slowness). ONE SHOULD ALWAYS EXAMINE THE GXXXX AND DXXXX FILES TO MAKE SURE ’G’ IS NOT SINGULAR, AND TO ADD ANY ADDITIONAL CONSTRAINTS . The program does try to prevent signularity by adding its own constraints (weighted by a factor of 9) which force delay times at near shots or receivers to the same value. For example, two geophones within the same ground consistent zone (3/4 average receiver or shot spacing) will be constrained to the same value.
The basic model is a single layer of overburden covering a single refracting interface. The Scilab procedures permit resolution of delay times into an earth model. Two types of resolution are possible; (1) as relative structural variation between the recording surface and the refractor, or (2) as relative variation in the overburden velocity. In both cases, the refractor is assumed to have a constant velocity. In case (1), the overburden velocity is assumed constant, in case (2), the distance between the recording surface and the refractor is assumed constant. In general, neither abstraction will be the truth. Rather these are end limit possibilites. If uncertain, the user is advised to invoke the first case. Unless significant overburden water content variations exist, case (1) seems to represent reality better than (2). There are no assumptions about the recording surface shape. The surveyed locations of sources and receivers are extracted from the seismic headers. Data do not even have to be acquired in a line, and 3-D distributions of sources and receivers are acceptable. The refraction setup REQUIRES that the offset range exclude direct arrivals. The minimum offset option is the way to block direct arrivals from being considered. The Scilab procedures for this analysis are delaytm.sci and delaytmplot.sci for common shot gathers, and delaytmR.sci and delaytmplotR.sci for common receiver gathers.
The basic model is velocity*time=distance. If the offset range excludes the refracted arrivals, multiple shot records may be analysed in a simultaneous least squares solution for overburden velocity. The direct wave setup REQUIRES that the offset range exclude refracted arrivals. The maximum offset option is the way to block refracted arrivals from being considered. The Scilab procedure for this analysis is direct.sci.
Options
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-h |
Online help giving details on command line arguments | ||
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line# |
4 Characters to identify the profile and label the matrices. | ||
|
nshots |
Number of shot (or receiver) gathers to be combined into a simultaneous inversion. | ||
|
rmin |
The minimum offset to include (can be used to exclude direct waves) | ||
|
rmax |
The maximium offset to include (can be used to exclude refracted waves) |
irefdir
Switch to select refraction or direct wave setup
0= refraction setup
1= direct wave setup
|
irecip |
Switch to select normal (shot gather) or reciprocal (receiver gather) case |
0= normal, common shot gathers
1= reciprocal,
common receiver gathers
infile_k
Input files (shot or receiver gathers), k=1,2,...nshots
NOTE:
If invoked with no options, will prompt user for input
parameters.
EXAMPLE:
bref 0001 3 40. 250. 0 0 k008.seg k009.seg k007.seg
Files k008.seg, k009.seg, and k007.seg will be jointly inverted for refraction analysis, and these are normal, shot gathers. Only offsets greater than 40 m and less than 250 m will be included in the analysis. Matrices will be labeled G0001 (system matrix), D0001 (rhs, time picks with station numbers), and E0001 (station numbers and recording surface elevations). After examining the G0001 matrix for singularity, and performing any editing, or the addition of constraints, the analysis will be performed in Scilab with delaytm.sci and nice plots may be made with delaytmplot.sci (exportable to xfig files for additional drafting).
|
Gline# |
System matrix. First nshot columns are for shots, next columns (one for each trace) are for the receivers, last column is source receiver offset (this would be for common shot gathers). | ||
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Dline# |
Right hand side of Gm=d, contains picked times, station numbers. | ||
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Eline# |
Contains trace number, station number, elevations; extracted from headers. |
standard output
lists common shot (or common receiver) coordinates as files are read.
brefxxxx.lst
echo check of input parameters, and lists average spacing and ground consistent zone.
bhelp(1), delaytm(1), delaytmR(1), delaytmplot(1), delaytmplotR(1), BSU users Guide
Michaels, P., (2001), "Use of Engineering Geophysics to Investigate a Site for a Bridge Foundation", in Foundations and Ground Improvement, T. L. Brandon editor, Geotechnical Special Publication No. 113, published by American Society of Civil Engineers, Reston, Va, p. 715-727.
Michaels, P., (1999), "Use of engineering geophysics in the design of highway passing lanes" in Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, SAGEEP99, Editors, M.H. Powers, L. Cramer, and R.S. Bell, EEGS, p179-187.
Michaels, P., (1995), A geophysical site investigation for a bridge foundation in a narrow canyon, Environmental and Engineering Geoscience, Vol. I, No. 2, pp.219-226.
No known bugs. WARNING: This program is intended for those trained in geophysical engineering and inversion theory. False and misleading results are likely if you are unfamiliar with either of these topics. You will most certainly have to manually edit the matrices, and this requires a background in the above topics.
Copyright © 2024 by Paul Michaels
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
P. Michaels, PE. <[email protected]>