Processed continuous resistivity profiling data collected in the Indian River Bay, Delaware, on April 13, 2010, on U.S. Geological Survey Field Activity 2010-006-FA

The continuous resistivity profile (CRP) system used on this cruise was an AGI SuperSting marine system described at the website: www.agiusa.com/marinesystem.shtml. The particular system used for this acquisition was a 50-m streamer with an 11 electrode array with electrodes spaced 5 meters apart. The source electrodes are graphite, while the receiver electrodes are stainless steel. A dipole-dipole configuration was used for the data collection in which two fixed current electrodes are assigned with the measurement of voltage potential between electrode pairs in the remaining electrodes. The maximum depth below the water surface the streamer can reach is approximately 1/4 the streamer length. So for the 50-m streamer, maximum depth is about 12.5 meters. Each line of data acquisition records several files. The two files necessary for processing are the *.stg and the *.gps file. The STG file contains the resistivity data, while the GPS file contains the navigation information. The navigation system used in concert with the CRP system is a Lowrance LMS-480M with an LGC-2000 GPS antenna and a 200 kHz fathometer transducer. The antenna and fathometer transducer were mounted on the starboard side of the boat. The streamer tow point was on the port side aft. The layback offset between the navigation antenna and the first electrode was 17.6 meters on April 13 and 14. On April 15 the antenna and transducer were moved 1.6 m aft changing the layback offset to 16 m. This layback offset is accounted for by the acquisition system. The approximately 2 m lateral offset is not accounted for. The Lowrance transducer also contains a temperature sensor. Lowrance indicates the speed of sound used by the system is 4800 feet/second. Both the temperature and depth information are recorded in the logged GPS file. There are instances where no depth or temperature information is recorded due to an equipment problem. The CRP system images the subsurface electrical properties of an estuarine, riverine or lacustrine environment. Resistivity differences can be attributed to subsurface geology (conductive vs less conductive layers) and hydrogeologic conditions with fresh water exhibiting high resistivity and saline conditions showing low resistivity.

Once the navigation and raw data were assessed to be okay, the actual processing of the resistivity data could start. Of note, see the metadata for jd103gps_bestdepth.shp (available at https://pubs.usgs.gov/of/2011/1039/html/ofr2011-1039-catalog.html) for a description of the work needed to extract valid bathymetry values for use in the data processing. The resistivity data were merged with the navigation data and linearized using AGI's Marine Log Manager (MLM) software. (Note that the Marine Log Manager version is different than the software version of the AGISSAdmin software of which it is a part - although shipped together, the software is developed separately). The GPS files used are the "newgps" files that have been modified to include best available bathymetry values. The version of Marine Log Manager used was AGI SSAdmin MLM v 1.3.4.217 The GPS offset was set to 0 meters since the offset (17.6 meters) between the navigation antenna and the first electrode of the resistivity streamer was accounted for in an acquisition offset. Lateral offset is not accounted for. The lines processed on this day are L1F1, L1F3, L1F5, L1F7, L2F1, L3F1, L4F1, L5F1, L5F2_part1, L5F2_part2, and L5F5. Two of these lines were split during the MLM processing due to sharp turns in the lines. Line L5F2_part1 was split into L5F2_part1a and L5F2_part1b and L5F5 was split into L5F5_a and L5F5_b. These line names are what the * refers to in the source used and source produced citations. This process step and all subsequent process steps were performed by the same person - VeeAnn A. Cross.

Each DEP file was checked for anomalous bathymetry values, or duplicated distance along values, and those lines in the file were deleted.

EarthImager software does not require that a default resistivity value for the water column be supplied in the DEP file. If one is not supplied, then it calculates a value based on the first electrode pair. On this survey, an additional instrument was deployed to measure water temperature and salinity. This instrument was a YSI 600XLM mounted just behind the GPS/fathometer. See the YSI metadata available at https://pubs.usgs.gov/of/2011/1039/html/ofr2011-1039-catalog.html for details on the processing of the YSI data. An average water resistivity value was calculated based on the YSI measurements during the time of collection of a given line of resistivity data collection. Note that the recorded YSI times are local times, while the CRP records UTC time. During this cruise, UTC is +4 hours compared to local time. This average resistivity value varied for each data file. The calculated value was added to the appropriate place in the DEP files so that a known water resistivity value would be used in the resistivity inversion calculations. These files were saved with a new filename with "wres" appended to the prefix of the filename indicating a water resistivity value is in the header of the file.

EarthImager version 2.2.8 build 562 was then used to process the data files. The process settings were set to CRP - Saltwater. The *.ini file accompanying the results contains the parameters used during processing. These parameters include: minimum voltage: 0.02; minimum abs(V/I): 2E-5; max repeat error: 3%; min apparent res: 0.03; max apparent res: 1000; max reciprocal error: 5%; remove negative resistivity, smooth model inversion; finite element method; Cholesky decomposition; Dirichlet boundary condition; thickness incremental factor: 1.1; depth factor: 1.1; max number of CG iterations: 100; stop criteria: number of iterations 8; max RMS 3%; error reduction 5%; L2Norm; CRP processing using a 65% overlap. These INI files can be loaded in EarthImager to help maintain consistent processing parameters for other datasets. When the files are processed, numerous files are generated. Because of the "roll-along" nature of the processing, each line takes several iterations of processing which are then combined into a single output. The output consists of numerous files including JPEG images and text files representing the XYZ position of each resistivity value. There are four JPEG images generated with each process when possible - a long version with the x-axis labeled with distance along line (in meters) and a corresponding short version of the same information. Additionally, there is a long version with the x-axis labeled with latitudes and longitudes and the corresponding short version with the same information. The JPEG files produced use a color scale for the resistivity that is based on the data extent from that particular file. The JPEG images also include a plot of temperature along the line. In addition to the JPEG images, there are text files with the extensions of *.llt, and *.xyz. Each of these is a text file. The LLT file has four columns of information: longitude in decimal degrees, latitude in decimal degrees, depth in meters, and resistivity value in ohm-m. The XYZ file has three columns of information: distance along line in meters, depth in meters, and resistivity value in ohm-m. There is also a file created with a UTM extension which has the same format as the LLT file except it contains the coordinates in UTM eastings and northings in the zone appropriate for the survey line (software determined). This file was not checked for accuracy as software versions in the past gave erroneous UTM fixes. An example of the file naming convention is as follows: For input files of L3F1_lin.stg and L3F1_lin_wres.dep the resulting series of output files are: L3F1_lin1_trial1.ini; L3F1_lin_AllInvRes.llt; L3F1_lin_AllInvRes.xyz; L3F1_lin_trial1_InvResLong.jpg; L3F1_lin_trial1_InvResShort.jpg. The JPEG images with distance along lines along the X-axis are L3F1_lin_trial1_InvResLong_linear.jpg and L3F1_lin_trial1_InvResShort_linear.jpg. You can process an individual line as many times as you want and the software places the results in incrementing folder names starting with trial1. These data represent trial3, which is the processing with the water resistivity value. The DEP file used in processing is also written to the results folder.

The XYZ output file was then loaded into MATLAB version 7.5.0.342 (R2007b), along with the depth information from the DEP file, to create a new JPEG image with the same color scale for all the data files. In this manner, the JPEG images can be compared directly. Care was taken to try to get the vertical and horizontal scales uniform as well, although this was not always possible due to MATLAB limitations. These images reside in the "matlabimages" folder. These JPEG images include a black line within the resistivity profile which represents the sediment water interface based on the depth values from the DEP file. The local MATLAB script used to load the data was cp_ir_50m.m, while the local MATLAB script used to export the JPEG image was exportfig.m.

Edits to the metadata were made to fix any errors that MP v 2.9.36 flagged. This is necessary to enable the metadata to be successfully harvested for various data catalogs. In some cases, this meant adding text "Information unavailable" or "Information unavailable from original metadata" for those required fields that were left blank. Other minor edits were probably performed (title, publisher, publication place, etc.). Attempted to modify http to https where appropriate. Moved the minimal source information provided to make it the first process step. The metadata date (but not the metadata creator) was edited to reflect the date of these changes. The metadata available from a harvester may supersede metadata bundled within a download file. Compare the metadata dates to determine which metadata file is most recent.

Added keywords section with USGS persistent identifier as theme keyword.