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U.S. Geological Survey Open-File Report 2005-1306

Continuous Resistivity Profiling Data from the Upper Neuse River Estuary, North Carolina, 2004-2005


Data Processing

Continuous Resistivity Profiling data were collected in the Neuse River Estuary in 2004 and 2005. Electrical surveys are conducted to determine resistivity measurements of the subsurface. The initial values recorded by the system are the measured apparent resistivity values. A 2D resistivity model, much like a 2D seismic survey, takes into account resistivity changes in the vertical and horizontal direction along a survey line, but assumes resistivity does not change in the direction that is perpendicular to the survey line (Loke, 1997). The data collected by a system is the apparent resistivity of the subsurface. These data then undergo an inversion process which proceeds to find the true resistivity profile which best produces the measured results. In the case of the EarthImager 2D software, the subsurface is divided into a number of rectangular blocks and the resistivity of these blocks is determined creating an apparent resistivity pseudosection that agrees with the measured apparent resistivity values. By constraining the model with the water depth profile, a more accurate resistivity profile can be generated. Processing parameters can be used to further constrain the modeling. The following paragraphs describe how the Neuse River data were handled, including the hardware, software, and processing parameters used.

processing flow diagram

Processing flow diagram.

Data were collected using an Advanced Geosciences, Inc. (AGI) SuperSting Marine system. This system was comprised of a multi-channel portable resistivity meter, the Marine Log Manager software, and a towed electrode cable. The depth of penetration was approximately 20% of the towed cable length. For the 2004 survey, the tow cable was 100 meters long and consisted of an array of 11 stainless steel electrodes spaced 10 meters apart. For the 2005 survey, the tow cable was 50 meters long and consisted of an array of 11 electrodes with electrodes spaced 5 meters apart. The potential electrodes on this cable were graphite, with the remaining electrodes stainless steel. A dipole-dipole configuration was used for the data collection in which two fixed current electrodes were assigned and voltage potentials were then measured between electrode pairs in the remaining electrodes. In addition to recording the apparent resistivity values into a raw STG file, the logging system was connected to a GPS enabled fathometer allowing the recording of position, water depth, and water temperature into a GPS file.

The Marine Log Manager software then allowed the recorded resistivity data to be edited and merged with the navigation data. Merging the navigation file with the resistivity data resulted in a linearized STG file. This file has a distance along line value for each electrode which is necessary for the inversion processing, i.e. linearizing the navigation positions. These data were then exported in a format compatible with EarthImager 2D inversion software. This export included two files; the linearized STG file which contained the resistivity information, and the DEP file which contained the bathymetric and water temperature information. Ancillary to the collection of the resistivity data was the collection of conductivity and temperature data with a YSI Multi-Parameter Water Quality Monitor or a WTW Multi-Parameter field meter. These data were used to derive an average water resistivity value for each survey line.

Some of the longer survey lines were broken into several parts. The primary reason for doing this was to eliminate turns from the processing of the data. Where applicable, survey lines from the two years were "split" so that where line coverage was duplicated, the breaks in the lines were the same.

When loading the data for processing in the EarthImager software, the DEP file is loaded in addition to the STG file. If the DEP file is not edited to supply an average water resistivity value for the water column, then the EarthImager software calculates a value based on the first electrode pair. What follows are examples of the DEP file header information, with the default water resistivity value and a supplied resistivity value.

default water resistivity measured water resistivity
; DEP File header
start=34.97233, -76.71340
stop=34.96647, -76.72972
;P1=1 (0.0)
;P2=336 (1712.7)
WaterRes=
unit=meters
; DEP File header
start=34.97233, -76.71340
stop=34.96647, -76.72972
;P1=1 (0.0)
;P2=336 (1712.7)
WaterRes=0.83
unit=meters

Both years of data were processed with the same software with the same parameters. In addition, each data file was processed with and without a measured water resistivity value. The software used was AGI's EarthImager 2D inversion software, version 1.9.0. The following series of screen captures portray the processing parameters used. Click on an image to see a full-size version of the image.

image of initial settings processing parametersInitial Settings tab image of IP Inversion processing parameters IP Inversion tab
image of forward modeling processing parameters Forward Modeling tab image of terrain processing parameters Terrain tab
image of resistivity inversion processing parameters Resistivity Inversion tab image of CRP processing parameters CRP tab

 

The EarthImager CRP (continuous resistivity profile) module is specifically designed to process large amounts of continuous resistivity data as is typically acquired in a marine survey. The strategy for the processing is a divide-and-conquer method in which the long section of a single collection file is divided into many subsections. These subsections are individually inverted and the processing culminates by assembling the individual sections into a single profile (Advanced Geosciences, Inc., 2005). All of these steps are saved into an individual folder. For the purposes of this data release, besides retaining the linearized STG and DEP files used as input for the processing, three files generated as a result of the processing are saved. These include:

  1. The smaller JPEG image of the complete resistivity profile. These images have been modified to remove the temperature profile and add text indicating the water resistivity value used.
  2. XYZ file in which x=distance along line, y=depth, and z=resistivity value.
  3. The first INI file which contains the processing parameters used. This INI file can be loaded in a subsequent file processing to ensure the same parameters are used for all files.

The JPEG images resulting from the EarthImager processing were saved with the default color scale used by the software. This color scale ranges from blues to reds with the reds being the more resistive values. Each individual image has a scale maximized for the range of resistivity values in that dataset.

In order to simplify the comparison of resistivity profiles, the XYZ file was combined with the DEP file in Matlab to generate JPEG images in which a common color scale was used for all files. For these images, the color scheme was inverted so that more resistive values are blue, with the more conductive values being red. Matlab was also used to plot the data in an attempt to show the JPEG images with a common scale both vertically and horizontally. Due to limitations with Matlab, this objective was not perfectly realized, but the relative plot sizes are more representative than the smaller EarthImager JPEGs - which used the same size image regardless of the number of electrodes measured and the corresponding variation in total lengths of survey lines. These Matlab plots will hopefully facilitate easier comparison of individual CRP profiles.

All of these JPEG profiles (both the EarthImager and Matlab versions) are available from this publication, as well as the raw and processed data files. These data are available from the Data Catalog page, with previews of the profiles available from the Preview pages.

Finally, a Visual Basic program was written to combine the linearized STG file with the DEP file to create a datafile in the RES2DINV format for users of that software package. These files are also available from the Data Catalog page.

Survey comparison

Parameter
2004 Survey
2005 Survey
tow cable length
100 meters
50 meters
electrode type
stainless steel
graphite
electrode spacing
10 meters
5 meters
approximate penetration
20-25 meters
10-15 meters
cable configuration
dipole-dipole
dipole-dipole
number of survey lines*
22
19
survey kilometers
108.2
97.4

*Number of survey lines is based on the number of resistivity lines processed. In cases where a single line is broken into multiple parts, each part is counted as a survey line.

 

 


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