DOI: 10.6075/j0rr1wj5
title: Environment Data (1996-05-10 to 1996-05-18). In SWellEx-96 Experiment Acoustic Data

description: Environment
The SWellEx test site has been extensively studied. An impressively detailed geoacoustic database and model exists for the region. In addition, a CTD survey was conducted during the SWellEx-96 Experiment to provide water column sound speed data.

Waveguide
For the waveguide (see component, "Waveguide plot"), the water depth is assumed to be the water depth at the array for range-independent processing. The seafloor is composed first of a 23.5 m thick sediment layer with a density of 1.76 g/cm^3 and a compressional attenuation of 0.2 dB/kmHz. The top of the sediment layer has a compressional sound speed of 1572.368 m/s while the bottom of the sediment layer has a compressional sound speed of 1593.016 m/s. Below the sediment lay an 800 m thick mudstone layer with a density of 2.06 g/cm^3 and an attenuation of 0.06 dB/kmHz. The top and bottom sound speeds of the mudstone layer are 1881 m/s and 3245 m/s, respectively. The geoacoustic model is completed by a halfspace with a density of 2.66 g/cm^3, an attenuation of 0.020 dB/kmHz, and a compressional sound speed of 5200 m/s.

Water Column Sound Speed
The CTD data from the SWellEx-96 Experiment was salinity-despiked and depth-interpolated every 0.5 m, yielding 51 final CTD casts. Each CTD data file contains five columns: (1) depth [m], (2) temperature [Deg C], (3) salinity (PSU), (4) sound speed (m/sec), and (5) sigma-t. Note: The times listed in the table component, "CTD locations" are local times (GMT - 7 hours). Add 7 hours to the listed time to convert to GMT. For a tarball containing ASCII versions of the CTD location information as well as the CTD data, select the component, "CTD data".

Bachman Database
Richard Bachman produced a seabed geologic model that is a gridded database containing water depth, sediment grain size, sediment thickness, and acoustic basement type. Grid cells are squares of side length equal to 2 arc seconds. The gridded database covers the area: 32 degrees 45 minutes to 32 degrees 33 minutes, and 117 degrees 26 minutes to 117 degrees 14 minutes.

Geographic data files are provided for: surface sediment grainsize (grainsiz.xyz), sediment thickness (sedthkms.xyz), basement rock type (basement.xyz), water depth (depth.xyz). Also needed is a sound speed profile, some are provided in the examples. Each data file has a companion README file explaining the units, derivation, and codes for land or null values (if any). The geographic data files ( *.xyz ) contain (X,Y,Z) triples, where X is the seconds west of 117 degrees 13 minutes west, and Y is seconds north of 32 degrees 32 minutes north. For example, if {x,y} = {-456,362}, the point is 456 seconds west of 117 degrees 13 minutes and 362 seconds north of 32 degrees 32 minutes.

Download the Bachman database by selecting component, "Bachman database". Note: The data herein was produced in September 1994, but represent the best information that was available for the period of Events S5 and S59.

The examples use a script call 'extract2.scr' that will take in a list of lat-long positions, will convert the positions into the corresponding arc second location in the database, and extract out the data corresponding to the lat-long position. There are three examples of extracting data from the bachman data base. All three examples call the script 'extract2.scr' to do the extraction. (Note: Matlab mapping toolbox required for range calculation.)

Example 1: The first example extracts the geo-acoustic data for the VLA location during SWellEx-96. The script to run this example is getVLA_GeoModel.scr. VLA location, 32 40.254 117 21.620, run through extract2.scr produces: 0.00, 219.73, 24.39, 5.05, 1.0 for columns 1-5, respectively, where: Column 1 = Range (m) from initial point, Column 2 = Water Depth (m), Column 3 = Two-way travel time in msec for sediment, Column 4 = Sediment Grain size, Column 5 = Basement Type (0-unknown, 1-Tertiary, 2-Cretaceous, 10-Land).

Example 2: The second example extracts a cut line along the longitude line 117 degrees 20 minutes (for NS track). This is a range independent track line. The script to run this example is plotBathyNS.scr.

Example 3: The third example extracts a cut line along the latitude line 32 degrees 39 minutes (for EW track). This is a range dependent track line. The script to run this example is plotBathyWE.scr.

bach2geo_MPL
Each of the Bachman database examples demonstrates how to extract data out of the Bachman geographic data base. But it does not produce a geoacoustic model. The document 'geo_modl.doc' explains how to construct a geoacoustic model from the output of the database. The program bach2geo_MPL implements the steps documented in 'geo_modl.doc'. There are two examples of how to use bach2geo_MPL.

Kraken Example: Using the results from Example 1, bach2geo_MPL is run to generate a kraken input. Kraken is then run 13 times (for 13 frequencies), with a MFP range-depth ambiguity surface computed from the average result of the 13 range-depth ambiguity surfaces.

RAM example: Using the results from Example 2, bach2geo_MPL is run to generate a RAM input file. The RAM input file is then used to produce a Transmission Loss plot at 201 Hz. Note that the computations to produce the geoacoustic model are dependent on the sound speed profile used as well as the results from the Bachman data base.

Download the bach2geo program and examples of how to run it by selecting component, "bach2geo program". Note this software is compiled for Red Hat Linux, but the source code is also present.