Time Variable Filtering
The Time Variable Filtering (TVF)
is a filtering technique that does not give a dispersion curve
but a smooth signal (a signal time-variable filtered), in which all effects of noise, higher
modes and other undesirable perturbations have been removed. This technique is a very useful
technique that allows obtaining a very good dispersion curve, from the seismic signal
time-variable filtered (Corchete et al., 2007).
The TVF is described with detail by Cara (1973). You can compute this
filtering technique by means of the present program, as it will be described below.
The TVF program and its files, which are needed to run this application, are enclosed into a ZIP
file named "tvf.zip". When you have got the ZIP
file and you have uncompressed this file, you have three files named tvf.exe,
dtvf.dat, grupo.dat and segnal.dat. The file tvf.exe contains a
program (in FORTRAN code for PC) for the computation of the TVF. This computer program will be
used to obtain the seismic signal time-variable filtered, from the pre-processed trace
contained in the file segnal.dat and the dispersion curve contained in the file
grupo.dat. The seismic signal time-variable filtered obtained with this program is
contained in the file named segfil.dat. Figure 1 shows the results of this program
(Corchete et al., 2007). All the program capabilities are controlled by parameters
enclosed in the file named dtvf.dat. This file is generated by the MFT program to be
used with the TVF program. Nevertheless, if you want generate your own file dtvf.dat, the
list of parameters is as follows:
N, NU, NVENT, ALFA, DIST
The description of all parameters is as follows:
N = Number of the records in the input file segnal.dat (maximum number = 16384).
NU = Number which value is given by the relationship N = 2**NU.
NVENT = Number which value is used to select a kind of window (Cara, 1973): (1) powers window,
(2) Tukey window and (3) cosine window.
ALFA = Parameter that controls the wide of the time-frequency window (Cara, 1973).
DIST = Epicentral distance (in this example the epicentral distance is 1208.9 km).
Fig. 1 (a) Observed seismogram corresponding to the pre-processed trace (instrument
corrected), contained in the file segnal.dat. (b) Group-velocity dispersion curve obtained
previously by application of MFT. (c) Time-variable filtered seismogram.
Cara M., 1973. Filtering dispersed wavetrains.
Geophys. J. R. astr. Soc., 33, 65-80.
Corchete V., Chourak M. and Hussein H. M., 2007.
Shear wave velocity structure of the Sinai
Peninsula from Rayleigh wave analysis. Surveys in Geophysics, 28, 299–324.