Processed Continuous Resistivity Profile Data Collected in the Corsica River Estuary, Maryland on May 16, 2007 on USGS Cruise 07005

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 ¼ 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 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 data could start. The resistivity data were merged with the navigation data and linearized using AGI's Marine Log Manager 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 version of Marine Log Manager used was AGI SSAdmin MLM v 1.3.4.201 The GPS offset was set to 0 since there is no offset between the navigation antenna and the resistivity cable tow point. The lines processed on this day are L15F1, L16F1, L17F1, L18F1, L19F1, L20F1, L21F1, L22F1, L23F1, L24F1, L24F2, L25F1, L26F1, L27F1 (parts 1 and 2), L28F1, L29F1, L30F1, L31F1, L32F1 (parts 1 and 2), L33F1, L34F1, L35F1, and L36F1. 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. Of note:L24F1, L24F2, L25F2, L26F1, L27F1_part1, L27F2_part2, and L28F1 have very sparse bathymetry information. These lines are handled differently in the remaining process steps. In addition, L24F1 and L27F1_part1 are very short lines, which will also make a difference in the details of the processing.

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. For this cruise instruments deployed in the field at the same time as the CRP data collection recorded water salinity and temperature. Averaging surface water and bottom water values yielded a salinity of 5.8 ppt and a temperature of 22 degrees Celsius. Based on these values, the calculated water resistivity value is 1.0345 ohm-m. This calculation was done using the website <http://www2.sese.uwa.edu.au/~hollings/pilot/denscalc.html>. This value was verified using the tool in VACExtras v. 2.1 - Resistivity - Calc resistivity. This value was added to the appropriate place in the DEP files so that a known water resistivity value would be used in the calculations.

EarthImager version 2.2.8 build 562 was then used to process the data files. The *.ini file accompanying the results contains the parameters used during the 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 image 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. In cases where a DEP is not used in the processing, only the long and short versions with the distance along line values along the x-axis are available. The JPEG files produced uses 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 was also a file created with a UTM extension, but the eastings and northing values were invalid so was omitted from this collection (software bug). 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. 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 trial2, which is the processing with the water resistivity value. In addition, the DEP (if used in processing) is written to the results folder. As noted earlier, some files had to be handled differently because of the lack of bathymetry. In these cases, the EarthImager processing was done without using the DEP file. The lack of actual bathymetry values caused the processing to crash if the DEP file was used in the processing. In the case of the extremely short files, no "roll-along" processing was required. In these cases, the JPEG images and the XYZ data had to be saved manually. First, just the inverted resistivity line is displayed using View - Inverted Resistivity Section. Then the image can be saved using File - Save Image. And finally, the XYZ data had to be saved manually using File - Save Data in XYZ format. The XYZ output file extension is DAT instead of XYZ.

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_corsica.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. Reordered the links in the identification section to have a landing page link as the first link. Updated the link to the field activity. Moved the minimal source information provided to make it the first process step. The distribution format name was modified in an attempt to be more consistent with other metadata files of the same data format. Added a distribution format to account for the JPEG images in the zip file. 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.