A MATLAB script (justbelowsed.m) was run on the processed CRP files to generate files with just the resistivity values that fall below the sediment water interface, and an extrapolated value at the sediment water interface. Two files from the processed resistivity data are needed for this script: the XYZ output from the EarthImager processing which has distance along line (meters), depth of resistivity reading (meters), and resistivity value (ohm-m); and the DEP file that contains distance along line (meters) and water depth values (meters). As noted in the vertical accuracy report, not all lines successfully collected fathometer data at the time of data collection and had to be further processed to derive bathymetric information. Also, because some lines are short and don't require a "roll-along" processing technique, the XYZ information is contained in a file with the DAT extension. The MATLAB script combines the two data files (processed resistivity and depth files) such that the output is only the resistivity values that fall at or below the sediment water interface. This script was written by the USGS (VeeAnn A. Cross) in Woods Hole, Ma. In order to have a resistivity value at the sediment water interface, the software usually has to interpolate resistivity values. The version of Matlab used for these data was MATLAB 220.127.116.112 (R2007b). An example of the script usage in MATLAB is:
The output is:
This resulting XYZ file has 5 columns of information: distance along line (meters); depth below water surface (meters); depth below sediment/water interface (meters); resistivity value (ohm-m); log(10) resistivity value. This process step and all subsequent process steps were performed by the same person - VeeAnn A. Cross.
See the metadata for the fixed bathymetry "fixbathyraw" available at: <https://pubs.usgs.gov/of/2010/1094/html/catalog.html> under the CRP data section for a full explanation of how bathymetry values were derived when none were collected in the field.