Multiple Filter Technique


Program objectives

The Multiple Filter Technique (MFT) is a filtering technique, which is used to obtain the group-velocity dispersion curve from a pre-processed trace (instrument corrected). This filtering technique is described in detail by Dziewonski et al. (1969). You can compute this filtering technique by means of the present program, as it will be described below.

Program description

The MFT program and its files, which are needed to run this application, are enclosed into a ZIP file named "mft.zip". When you have got the ZIP file and you have uncompressed this file, you have three files named mft.exe, dmft.dat and segnal.dat. The file mft.exe contains a program (in FORTRAN code for PC) for the computation of the MFT. This computer program will be used to obtain the group-velocity dispersion curve from the pre-processed trace contained in the file segnal.dat. The program requires the input of the epicentral distance during its execution. 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 dmft.dat. This file is in the free format and contains the parameters:

N, NU, TMAX, TMIN, NF, ALFA, NMAP

The description of all parameters is as follows:

N = Number of the records in the input file segnal.dat (maximum number = 1048576 = 2**20).
NU = Number which value is given by the relationship N = 2**NU.
TMAX = Maximum value of the period to be obtained for the dispersion curve.
TMIN = Minimum value of the period to be obtained for the dispersion curve.
NF = Number of the equidistant periods to be obtained between TMIN and TMAX (maximum number = 256).
ALFA = Parameter that controls the gaussian filter (Dziewonski et al., 1969).
NMAP = With value 1 a data file (named mapa.dat) is prepared to plot the contour map, as it is shown in Figure 1b. The group-velocity dispersion curve obtained from the pre-processed trace (contained in the file segnal.dat) is contained in the file named grupo.dat. A new data file named dtvf.dat is also generated to be used with the TVF program. With value 0 the data file mapa.dat is not prepared, speed up the running of the program.

Fig. 1 (a) Pre-processed trace contained in the file segnal.dat. (b) Contour map of relative energy normalized to 99 decibels, as a function of period and group time (white line denotes the group times inferred from the energy map). (c) Group-time dispersion curve inferred from the energy map. (d) Group velocities calculated from the group times and the epicentral distance (in this example the epicentral distance is 1208.9 km). The group velocity is the epicentral distance divided by the group time, for each period.

References

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, 299324.

Dziewonski A., Bloch S., and Landisman M., 1969. A technique for the analysis of transient seismic signals. Bulletin of the Seismological Society of America, 59, No. 1, 427-444.