MB-System Unix Manual Page
Section: MB-System 5.0 (1)
Updated: 24 December 2009
mbsegypsd - Calculates the power spectral densisty function (PSD) of each trace in a
segy file, outputting the PSD estimates as a GMT grid file with trace number along
the x axis and frequency along the y axis.
mbsegypsd -Ifile -Oroot
[-Ashotscale -Ddecimatex -R
-Wmode/start/end -H -V]";
mbsegypsd calculates the power spectral densisty function (PSD) of each
trace in a segy file, outputting the PSD estimates as a grid with
trace number along the x axis and frequency along the y axis.
The output files are GMT netCDF format grid files.
The x-dimension of the grid is determined by the number of traces specified
by the -S option and any decimation specified with option -D.
If the -S option is not specified, then all of the traces in the segy file
will be processed. If the time sweep and delay (if any) are not specified using
the -T option, then the sweep and delay will be set so that all trace samples
are incorporated into the grid.
The y-dimension of the grid is determined by the sample interval in the time series
data. The frequency range runs from zero Hz to a maximum frequency given
in Hz by 1/(2*sampleinterval), where the sampleinterval is in seconds. So,
for instance, hydrophone data sampled at 26 kHz (26000 samples/second) will
have a sample interval of (1/26000 = 0.00003846 seconds). In this case, the
PSD will be calculated for frequencies ranging from 0 Hz to 13000 Hz (13 kHz).
For each trace, the PSD will be calculated by ensemble averaging, which means that
the trace will be broken up into a number of segments of length nfft (specified
with the -N option, default = 1024), a fast-Fourier-transform (FFT) will be
calculated for each segment, and the PSD estimate will be the average of
the FFT magnitudes over all segments. Before the FFT is calculated, each segment
is multiplied by a Hanning (cosine) taper; the PSD estimates are normalized to
account for the loss of signal due to the taper application.
The output grid can be either in linear units (Intensity/Hz) or in log-scaled dB/Hz
calculated as 20 * log10 (raw-PSD-value). The default is the former, and the
-L option causes output in the dB/Hz form.
A shellscript invoking GMT programs to plot the PSD grid is automatically
David W. Caress (firstname.lastname@example.org)
Monterey Bay Aquarium Research Institute
Dale N. Chayes (email@example.com)
Lamont-Doherty Earth Observatory
This option causes the x-axis to be rescaled from shot number to distance in meters.
The shotscale value represents the shot spacing in meters.
Sets the decimation of traces (decimatex) used in generating the output grid.
This "help" flag cause the program to print out a description
of its operation and then exit immediately.
Sets the filename of the input segy seismic data file to be gridded.
Sets the PSD grid output to be in dB/Hz.
Sets the filename root for the output GMT netCDF format grid.
This option sets the range of traces that are gridded, and thus determines
the x-dimension of the output grid (also impacted by any decimation
specified with -D). If mode = 0, then start and end
refer to shot numbers. This typically is useful for subbottom data or seismic
data in shot gather form. If mode = 1, then start and end
refer to CMP (or RP or CDP) numbers. This typically is useful for seismic
reflection data in stacked or CMP gather forms. If the data are multichannel
seismic reflection or seismic refraction in either shot or CMP gathers, the
start and end of the channels selected for gridding is set using the optional schan
and echan, respectively. The x-dimension of the output grid is determined by
(end - start + 1) * (echan - schan + 1) / decimatex.
The sweep value sets the time range of seismic data to be processed in seconds.
The optional delay value sets the sweep start time, again in seconds.
Normally, mbsegypsd prints out information regarding its
controlling parameters during execution; the
-V option causes the program to also print out statements
indicating its progress.
This option can be used to limit the data being processed to a particular time
window in various ways. This option does not impact the definition of the overall
grid bounds, but does restrict the data processed to samples within particular times
of interest. If mode = 1, then start and end are simply start and end
times of good data in seconds. If mode = 2, then start and end are relative to
the time of the bottom return. In this case start is often negative so that the
grid shows data above the seafloor, and then down into the subsurface. Finally, if mode = 3,
then start and end are relative to the time corresponding to the sonar
Suppose that one has a segy file of hydrophone data sampled at a
37 microsecond interval, corresponding to a 27027 Hz sampling rate.
Each of 1610 traces consists of 64865 samples. In order to create a
sonogram displaying the power spectral density as a function of
time, execute mbsegypsd as follows:
mbsegypsd -I 20090922_1833_V3422_S7_CH025ms.segy -N1024 -L -O testpsd -V
Here the -N option sets the FFT dimension used to be 1024 samples, which
means that the PSD estimate for each trace is the average of 64865 / 1024 = 63
calculations. The shell output of the program is:
Version $Id: mbsegypsd.1 1938 2012-02-22 20:58:08Z caress $
MB-system Version 5.1.2beta12
Input segy file: 20090922_1833_V3422_S7_CH025ms.segy
Output fileroot: testpsd
trace mode: 0
trace start: 0
trace end: 1609
channel start: 1
channel end: 1
trace decimation: 1
time sweep: 2.399968 seconds
time delay: 0.000000 seconds
sample interval: 0.000037 seconds
window mode: 0
window start: 0.000000 seconds
window end: 0.000000 seconds
grid filename: testpsd.grd
psd filename: testpsd_psd.txt
x grid dimension: 1610
y grid dimension: 513
grid xmin: -0.500000
grid xmax: 1609.500000
grid ymin: -13.171066
grid ymax: 13500.342448
NaN values used to flag regions with no data
PROCESS read:0 position:0 shot:0 channel:1 2009/265 18:32:59.000 samples:64864 interval:37 usec minmax: -102498.789062 -187.008667
PROCESS read:25 position:25 shot:25 channel:1 2009/265 18:34:14.000 samples:64864 interval:37 usec minmax: -201902.890625 9166.462891
PROCESS read:50 position:50 shot:50 channel:1 2009/265 18:35:29.000 samples:64864 interval:37 usec minmax: -203695.765625 28316.906250
PROCESS read:75 position:75 shot:75 channel:1 2009/265 18:36:44.000 samples:64864 interval:37 usec minmax: -201434.375000 24819.115234
PROCESS read:100 position:100 shot:100 channel:1 2009/265 18:37:59.000 samples:64864 interval:37 usec minmax: -16544.123047 37.462040
PROCESS read:1500 position:1500 shot:1500 channel:1 2009/265 19:47:59.000 samples:64864 interval:37 usec minmax: -8367.179688 -876.420044
PROCESS read:1525 position:1525 shot:1525 channel:1 2009/265 19:49:14.000 samples:64864 interval:37 usec minmax: -9163.580078 201.037201
PROCESS read:1550 position:1550 shot:1550 channel:1 2009/265 19:50:29.000 samples:64864 interval:37 usec minmax: -12128.291016 -993.766357
PROCESS read:1575 position:1575 shot:1575 channel:1 2009/265 19:51:44.000 samples:64864 interval:37 usec minmax: -67764.585938 2457.883789
PROCESS read:1600 position:1600 shot:1600 channel:1 2009/265 19:52:59.000 samples:64864 interval:37 usec minmax: -190216.031250 -14304.720703
The output PSD grid file is named testpsd.grd. A shellscript named
testpsd.grd.cmd is also created that, when executed, will generate a plot of the
sonogram. MBsegypsd also outputs an ascii file containing the average PSD for the
entire segy file in the form of frequency PSD pairs. In this case, the average PSD file
is named testpsd_psd.txt, and has contents like:
A shellscript named testpsd_psd.txt.cmd is created that will, when executed, generate
a plot of the average PSD.
mbsystem(l), mbm_grdplot(l), mbmosaic(l), mbm_grid(l)
The plots could be better...
- SEE ALSO
Last Updated: 24 December 2009
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