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Knudsen Engineering 320 Echosounder

  • October 2006 Sea Technology article pg1 pg2 pg3 - "Chirp Sub-Bottom Profiler Processing - A Review"
  • Notes - Paul's review of sub-bottom profilers.

                                                 Document date:  5 September 2008

    Processing Knudsen SEG-Y correlate files

    See below for a comparison of processing through PostSurvey and SIOSEIS. These scripts require several programs: sioseis - Preferably ver 2009.1.5 (13 Jan 2009) or newer. lsh - A sioseis program to list the start and end dates of a SEG-Y file. sio2sun - A sioseis program to convert Sioseis Versatec rasterfiles to Sun rasterfiles. convert - A ImageMagick program that converts plot file formats. display - A ImageMagick program that displays plot files.

    Convert all Knudsen sgy files in a directory to SEG-Y depth files

    raw2depth input-directory output-directory [EDEPTH] - Convert ALL raw (correlates) files in a directory to envelopes in time and depth. [EDEPTH] is optional and the units are meters. The preset is 16000. All depth traces start with zero depth and have the same length. (The time domain files have varying deep water delays and sample intervals.) The sample interval of the depth trace is 1 (meter) if EDEPTH is > 8192, .5 when 4095 < EDEPTH < 8192, .25 when 2047 < EDEPTH < 4096, .125 when EDEPTH < 2048. env2depth input-directory output-directory [EDEPTH] - Convert ALL envelope files in a directory to depth. EDEPTH is as in raw2depth. plot_depth depth-filename - Create a png file and display the plot of a file. September 2008 usage. - Comparison of Sioseis-segy and PostSurvey-keb processing.

    General purpose single file scripts

    mkenv - Make an envelope file from a correlate file. mkplt - Make a png plot file from an envelope file. disko - Create a SEG-Y file so that all traces start at time zero and are 8 seconds in length, with a sample interval of .001. mkdplt - Create a png plot in depth from an envelope file.

    Convert ALL files in a directory

    mkenv-all - Make envelope files from correlate files. Converts correlates to envelopes, applies a 2x500 hertz bandpass filter and decimates by a factor of 8. The process of making envelopes reduces the frequency content of the data to below 500 hertz. The output sample rate thus may be 1kHz (sample of 1 mil or .001 second). e.g. mkenv-all /Volumes/SeismicArchive/2003/CNTL03RR/Knudsen /Users/henkart/data/knudsen/env This script converts all the .sgy files in directory /Volumes/SeismicArchive/2003/CNTL03RR/Knudsen to envelope files in directory /Users/henkart/data/knudsen/env. The envelope filenames are the same as the correlate filenames with the prefix "env-". e.g. 176>ls /Users/henkart/data/knudsen/env env-0001_1809_LF.sgy env-0003_1013_LF.sgy env-0005_0057_LF.sgy env-0003_0501_LF.sgy env-0004_1110_LF.sgy env-0006_1325_LF.sgy env-0003_0610_LF.sgy env-0004_1217_LF.sgy info env-0003_1007_LF.sgy env-0004_1408_LF.sgy info-sorted The output directory also contains a file, "info-sorted", with a sorted list of the file start and end times and data trace start and end times. e.g. 177>more /Users/henkart/data/knudsen/env/info-sorted env-0001_1809_LF.sgy Begins: day 6 18:09:10 Ends: day 6 18:46:25 data tim es: 0.000 to 0.067 secs. env-0003_0610_LF.sgy Begins: day 7 06:10:11 Ends: day 7 08:32:29 data tim es: 1.866 to 2.667 secs. env-0003_1007_LF.sgy Begins: day 7 10:07:43 Ends: day 7 10:10:57 data tim es: 2.400 to 2.667 secs. env-0003_1013_LF.sgy Begins: day 7 10:13:21 Ends: day 7 11:26:35 data tim es: 1.200 to 2.667 secs. env-0003_0501_LF.sgy Begins: day 8 05:01:05 Ends: day 8 05:01:12 data tim es: 1.333 to 4.000 secs. env-0004_1110_LF.sgy Begins: day 9 11:10:24 Ends: day 9 12:17:26 data tim es: 2.000 to 2.667 secs.

    Create a png plot file for each envelope file in a directory.

    mkplt-all - Make png plot files from envelope files. mkplt-all is intended as a "quick look" plot by using sioseis plot parameter CHART, which mimics a chart strip recorded. CHART forces a "window" change so that the data remain in the plot window - the time origin of the plot changes. This script requires SIOSEIS ver 2008.6.6 (24 June 2008) or newer, The script only plots the first 5000 pings so that the plot doesn't get too large. The Knudsen program that converts KEB files to SEG-Y has/had a bug that wrote the water depth byte swapped. Modify the plt scripts if correct this if your data were created through this program.

    Create a single png plot file for a single envelope file.

    mkplt - Make a png plot files from an envelope file.

    Making all traces have the same length and starting at zero.

    Some non-sioseis programs are fussy about sample intervals and non-zero start times (SEG-Y deep water delay). These programs may require that the data start at time zero and every trace must be the same length. Some programs (SU) require every trace start at the same start time. The Knudsen SBP only records a small "window" of data - basically what is seen on the realtime screen. When recording SGY data, the window is NOT adjusted automatically for variable water depth. The watch must adjust the window so that the water bottom stays in the window. Sioseis processes DISKOX and DISKIN parameter SET may be used to control the start and end times of each trace. Zeroes will used to pad the data, thus increasing the length of each trace. This however can create traces with too many samples for the SEG-Y format (32768 samples). (Some programs many not be able to read more than 16384 samples per trace). Sioseis process RESAMP will resample data to a new sample interval through interpolation. The example script assumes the input data have been filtered previously to prevent aliasing. The example script creates a dataset of constant start time of 0 and end time of 8 seconds with a sample interval of .001 seconds (1 mil).

    Comparison of different recording and processing methods

    This dataset was recorded both KEB and SGY files on an SIO ship. Files 2007_171_1701.keb and 2007_171_1701_LF.sgy were recorded on the ship. File 2007_171_1701conv.sgy was produced from the keb file using PostSurvey. 171_keb.bmp was produce by PostSurvey using 2007_171_1701.keb as input and croppped and enlarged with Photoshop. env-2007_171_1701_LF.png was produced by Sioseis using scripts mkenv and mkdplt. mkdplt uses a smaller exponential gain (4 vs 5) than the scripts above. 171_converted.bmp was produced by PostSurvey using file 2007_171_1701conv.sgy (produced by PostSurvey from 2007_171_1701.keb). U_2007_171_1701.keb.sgy is a simple plot of 2007_171_1701conv.sgy. It appears that this file has been converted to envelope, but not depth. Note that the delay and time scale are incorrect. 171_segy.bmp was produced by PostSurvey using 2007_171_1701_LF.sgy (as recorded). This plot appears to be of the raw, unprocessed, correlates.
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