Processed Continuous Resistivity Profile Data Collected in the Potomac River/Chesapeake Bay on Sept. 6, 2006

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. The GPS offset was set to 7.9 meters to account for the difference between the navigation antenna and the resistivity cable tow point. The lines processed on this day are L1F1, L2F1, L3F1, L4F1, L5F1, and L6F1. 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.

The 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. For this cruise, another system was acquiring data at the same time as the resistivity system. I took the data from that system which included conductivity and temperature measured values of the near surface water and reformatted these data text files that could be loaded into ArcMap as XY Event themes. The reformatting was accomplished with a locally written python script called r_nav.py.

These text files were then loaded into ArcMap 9.0 using Tools - Add XY data and setting the projection to Geographic, NAD83. Once the event layers were loaded into ArcMap they were converted to shapefiles using the Data Export option. The individual shapefiles were then merged into a single shapefile representing the Julian Day using ET GeoWizards version 9.

I then used the resistivity calculator in VACExtras v. 1.95 to convert the temperature and salinity values to resistivity values with the units of ohm-m. The result of the calculation is to add an attribute to the shapefile called "resval". In the event that either the temperature or salinity value is not available, a resval of -9999 is used as the calculated value.

I then used the ArcToolbox 9.0 Analysis tools - Proximity - Near tool with the following parameters: input feature - the points containing the calculated water resistivity values; near features - the resistivity tracklines from this particular Julian day; search radius - 40 meters. Running this tool added the fields NEAR_FID and NEAR_DIST to the input point shapefile containing the water resistivity values (jd249_pnts.shp). I chose a 40 meter radius because I wanted to exclude far away values so as not to skew results. The instrument collecting the water conductivity information does not track right on top of the CPR navigation, so comparing the two datasets indicated a 40 meter search radius was appropriate.

I then created a text field called "resline" within the point shapefile using the attribute table option - Add field. I then joined the point shapefile to the polyline shapefile using the NEAR_FID attribute in the point shapefile and the FID attribute from the polyline shapefile. I selected all records within the joined table where NEAR_FID <> -1. the -1 is what is put for a point that doesn't fall near a line within the search radius (in the Near tool). I then copied over the line name attribute value from the polyline shapefile to the resline attribute within the point shapefile.

By selecting all the records for a given CRP line I can do the statistics to calculate the mean of the calculated water resistivity values. First I use the properties of the shapefile to define a definition query that will exclude all resistivity values that equal -9999 since this is essentially a NODATA value. Once this is done, the mean value is then used as the water resistivity value within the DEP file for that particular CRP line. This value is added to the appropriate location within the DEP file, and this file is then saved as *_lin_wres.dep where the * refers to the original line name. (* on this day refers to L1F1, L2F1, L3F1, L4F1, L5F1 and L6F1).

EarthImager version 1.9.9 was 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 spikes, smooth model inversion; finite element method; Cholesky decomposition; Dirichlet boundary condition; thickness incremental factor: 1.1; depth factor: 1.1; number of iterations: 8; stop criteria: 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 two JPEG image generated with each process - a long version and a short version. 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 L1F1_lin.stg and L1F1_lin_wres.dep the resulting series of output files are: L1F1_lin1_trial1.ini; L1F1_lin_AllInvRes.llt; L1F1_lin_AllInvRes.xyz; L1F1_lin_trial1_InvResLong.jpg; L1F1_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. In addition, the STG and DEP used as input to the processing software are 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_100m_potomac.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 2.9.34 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.). The source information was incomplete and had to be modified to meet the standard. The distribution format name was modified in an attempt to be more consistent with other metadata files of the same data format. 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.