MB-System Unix Manual Page


Section: MB-System 5.0 (1)
Updated: 3 June 2013


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.



Version 5.0



mbsegypsd -Ifile -Oroot [-Ashotscale -Ddecimatex -R -Smode[/start/end[/schan/echan]] -Tsweep[/delay] -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 generated.



David W. Caress (caress@mbari.org)

  Monterey Bay Aquarium Research Institute
Dale N. Chayes (dale@ldeo.columbia.edu)

  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 depth.



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:

        Program mbsegypsd

        Version $Id: mbsegypsd.1 2096 2013-06-04 08:17:22Z caress $

        MB-system Version 5.1.2beta12

        MBsegypsd Parameters:

        Input segy file:         20090922_1833_V3422_S7_CH025ms.segy

        Output fileroot:         testpsd

        Input Parameters:

             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

        Output Parameters:

             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

             shotscale:          1.000000

             frequencyscale:     1.000000

        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

 .br 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:

        0.000000 193.762464

        26.342132 99.114274

        52.684263 93.781880

        79.026395 86.775795

        105.368526 80.297280

        131.710658 74.908340

        158.052790 71.366137

        184.394921 70.319082

        210.737053 69.840534

        237.079184 70.794517

        263.421316 72.002905

        289.763448 71.979001

        316.105579 70.637717

        342.447711 69.652364


        13329.118592 -13.659568

        13355.460724 -13.722928

        13381.802855 -12.981740

        13408.144987 -12.626286

        13434.487119 -12.533222

        13460.829250 -13.659002

        13487.171382 -24.474310

A shellscript named testpsd_psd.txt.cmd is created that will, when executed, generate a plot of the average PSD.



mbsystem(1), mbm_grdplot(1), mbmosaic(1), mbm_grid(1)



Probably... The plots could be better...




Last Updated: 3 June 2013

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