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Exercises 1 and 2 Purpose - create synthetic seismograms for use in group velocity and phase velocity analysis.

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Presentation on theme: "Exercises 1 and 2 Purpose - create synthetic seismograms for use in group velocity and phase velocity analysis."— Presentation transcript:

1 Exercises 1 and 2 Purpose - create synthetic seismograms for use in group velocity and phase velocity analysis

2 Data Sets For simplicity, each group will login as temporary user 1 Each group will create have a subdirectory, e.g., group01, group02 etc Assume that you are in group01 cd group01 gunzip -c /mount/cdrom/usr/tutorial.tgz | tar xvf - or tar zxvf /mount/cdrom/usr/tutorial.tgz This creates the folder EASA462 with subfolders for the exercises.

3 Exercise.01 cd exercise.01 ls DOIT creates synthetics and invokes analysis DOCLEAN cleans up the area of temporary files We will now examine the 166 lines of the script

4 ##### # create the velocity model interactively ##### mkmod96 << EOF [Emulates terminal input to create the file simple.mod simple.mod ] Simple Crustal Model EOF

5 ##### # create the dfile use sprep96 -h for the format ##### cat > dfile << EOF EOF [The #’s are comments. This will create a synthetic at an epicentral distance of 2500 km, with a sample interval of 0.5 sec, 2048 points, with a first sample that starts at the time t0 = 0 + dist(km) / 8.0(km/s) ]

6 ##### # setup multimode surface wave dispersion and synthetics ##### HS=10.0 HR=0.0 sprep96 -M simple.mod -d dfile -L -R -NMOD 100 -HS ${HS} -HR ${HR} ##### # get the dispersion and the eigenfunctions ##### sdisp96 sregn96 slegn96 [Source depth of 10 km, Love and Rayleigh, 100 modes, get dispersion, get Love and Rayleigh wave eigenfunctions

7 ##### # make the synthetic for the station for a given # focal mechanism and then convert to SAC ##### DIP=45 RAKE=45 STK=45 AZ=45 BAZ=225 MW=4.5 spulse96 -p -V -l 4 -d dfile | \ fmech96 -D ${DIP} -R ${RAKE} -S ${STK} -A ${AZ} -ROT -MW ${MW} | \ f96tosac -B The \ indicates that the next line is a continuation. We create a synthetic and convert it to a SAC format. In this case there will be three SAC files, for the Z, R and T components of motion at the receiver. The use of SHELL variables is introduced to show how the code can be generalized, e.g., simpler to change the STK=45 line than to remember that the argument of -S is the strike.

8 ##### # convert synthetic output to meters/sec from cm/sec ##### gsac << EOF r *sac div 100 w q EOF Here we change the units of the SAC files for consistency with how real data are handled.

9 ##### # do interactive multiple filter analysis to # get group velocity dispersion ##### do_mft *.sac This will be done interactively. We will create some group velocity dispersion files. To see what is done, use two windows. MFTSRF *.dsp > disp.d Convert do_mft format to that required by the dispersion program, e.g., to surf96 format.

10 ##### # get the theoretical dispersion for this model ##### sdpegn96 -R -S -U -D disp.d -XLOG -PER sdpegn96 -L -S -U -D disp.d -XLOG -PER ##### # Look at the observed and predicted disperion ##### plotxvig < SREGNU.PLT plotxvig < SLEGNU.PLT Since the data set is created theoretically, we can always compare the observed and predicted dispersion. This illustrates the imperfection in determining group velocities. To see other problems, try STK=0, DIP=90, RAKE=0,AZ=22 and HS=20 - this will give a big spectral hole in the Rayleigh wave

11 ##### # create a test model and then invert using surf96 # note that we can not put this in the following interactive # script because of the way the CYGWIN disconnects subprocesses ##### cat > sobs.d << EOF modl.in starting model disp.d dispersion data set EOF This is the control for surf96. Normally it is set up interactively.

12 cat > modl.in << EOF MODEL.01 surf96 inversion model ISOTROPIC KGS FLAT EARTH 1-D CONSTANT VELOCITY LINE08 LINE09 LINE10 LINE11. H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS EOF Set up initial model which is a halfspace model. No assumptions about moho

13 ##### # now run the surface wave from the command line ##### # fix the halfspace velocity ##### surf For stability ##### # invert ##### surf Do 6 iterations surf ##### # save the model ##### surf96 28 modl.out

14 ##### # manually plot the fit to the dispersion # and the resolution kernels ##### srfphv96 srfphr96 plotxvig < SRFPHV96.PLT plotxvig < SRFPHR96.PLT ##### # overlay the observed and predicted models ##### shwmod96 -LEG -K -1 simple.mod modl.out plotxvig < SHWMOD96.PLT Shows first use of CALPLOT graphics. To get a PostScript file plotnps -F7 -W10 -K file.ps

15 Exercise.02 cd exercise.02 ls DOIT creates synthetics and invokes analysis DOCLEAN cleans up the area of temporary files We will not examine the 92 lines of the script

16 Create synthetics from 2500 to 2850 km with 50 km spacing. This is typical of some arrays. Determine phase velocities by performing a phase velocity analysis Compare phase velocities determined using do_pom to theoretical values

17 Summary Creating synthetics for an appropriate velocity model assists in the design of a field experiment or the choice of processing parameters Inversion programs have many controls. A model that fits the data is not necessarily the true Earth model

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