Continuous Resistivity Profiling

Continuous Resistivity Profiling (CRP) data were collected in the waters of Cape Cod National Seashore in 2004 and 2006 (Table 2).

Surveys are conducted to determine electrical resistivity of the subsurface (Manheim et al., 2004; Cross et al., 2006). The initial values recorded by the system are measured apparent resistivity values. A 2D resistivity model takes into account resistivity changes in the vertical and horizontal direction along a survey line, but it is assumed that resistivity does not change in the direction that is perpendicular to the survey line (Loke, 2000). The apparent resistivity data undergo an inversion process, which produces the actual resistivity profile that would most likely yield the measured values. In the case of EarthImager 2D inversion 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 is consistent with the measured apparent resistivity values. By constraining the model with the water-depth profile, where water thickness (depth) and resistivity are known from field measurements, a more accurate subsurface-resistivity profile can be generated. Processing parameters can be used to further constrain the modeling. A description of how the CCNS data were handled follows (Figure 5), including the hardware, software, and processing parameters used. The data, both raw and processed, including Federal Geographic Data Committee (FGDC) compliant metadata, can be accessed through the GIS/Data Catalog page.

Figure 5: Resistivity data flow diagram.

Data were collected by using an Advanced Geosciences, Inc. (AGI) SuperSting Marine system. This system comprised a multichannel portable resistivity meter, the Marine Log Manager software, and a towed floating electrode cable. The depth of penetration was approximately 20 percent of the towed cable length. For the 2004 survey, the towed cable was 100 meters long and consisted of an array of 11 stainless steel electrodes spaced 10 meters apart. For the 2006 survey, the towed cable was 50 meters long and consisted of an array of 11 electrodes spaced 5 meters apart. The potential electrodes on this cable were graphite, with the remaining electrodes made of 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 Global Positioning System (GPS) receiver. In 2004, fathometer data were recorded to separate text files. These files had no time stamps, but the start and stop of the fathometer recording were coincident with the start and stop of the corresponding resistivity data logging (including the GPS data). On the basis of this assumption, the appropriate fathometer readings were extracted from the text file, converted to meters, and merged with the GPS file in the format required by the Marine Log Manager software. Two files (nauset1 and nauset2) are the exceptions, in that the fathometer data spanned both files and had to be split between the two. In 2006, the resistivity logging system was connected to a GPS-enabled fathometer, allowing for the recording of position, water depth, and water temperature in the 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 value called "distance along line" for each electrode that is necessary for the inversion processing,that is, 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 bathymetry and, in 2006, the water-temperature information. Also in 2006, ancillary to the collection of the resistivity data was the collection of conductivity and temperature data with a YSI Multi-Parameter Water Quality Monitor. These data were used to derive an average water-resistivity value for each 2006 survey line.

When the data for processing is loaded in the EarthImager 2D 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 2D software calculates a value based on the voltage signal from the first electrode pair on the survey streamer. For the 2004 data, no resistivity value was supplied, whereas in 2006 a measured value was supplied. Examples of the DEP file header information from each survey year are shown in table 3.

Data from surveys conducted in both 2004 and 2006 were processed with the same software (although different versions) but slightly different parameters as a function of the software version used. The 2004 data were processed with EarthImager 2D version 1.7.8, whereas the 2006 data were processed with version 2.01. A series of screen captures (table 4) portrays the processing parameters used for the 2006 data. Click on a table-cell thumbnail to see a full-sized version of the image.

The EarthImager 2D CRP module is specifically designed to process large amounts of continuous resistivity data as are typically acquired in marine surveys. The strategy for the processing could be described as 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 report, besides retaining the linearized STG and DEP files used as input for the processing, three types of files generated as a result of the processing were saved. These include:

1. JPEG images of the complete resistivity profile. These images have been modified to remove the temperature-profile plot that the software produces. Both long and short versions are available for each line. For the 2006 data, four JPEG images were generated by EarthImager 2D for each data file: a long and short version with distance values along the X-axis, as well as a long and short version with latitude/longitude values along the X-axis.
2. Text files of the processed resistivity data. For 2004 and 2006 data these include XYZ files in which X = distance along line, Y = depth, and Z = resistivity value. Due to software changes, the processed data from 2006 also have LLT and UTM files for each processed line. The LLT file contains longitude, latitude, depth, and resistivity values. The UTM (Universal Transverse Mercator) file contains easting, northing, depth, and resistivity values.
3. The first INI file which contains the processing parameters used. This INI file can be loaded prior to subsequent file processing to ensure that the same parameters are used for all files or to verify what processing parameters were used.

The JPEG images resulting from the EarthImager 2D processing were saved with the default color scale generated by the software. This color scale ranges from blues to reds with the reds representing the higher resistivity values corresponding to fresher ground water. Each individual image has the scale maximized for the range of resistivity values in that data set.

In order to facilitate the comparison of different resistivity profiles, the XYZ file was combined with the DEP file by using Matlab software to generate JPEG images in which a common color scale was used for all the files. For these images, the polarity of the color scheme is the same as that of the EarthImager 2D JPEGs in that the colors range from blue to red with the reds indicating higher resistivity values. Matlab software was also used to plot the data in an attempt to show the JPEG images with a common distance scale, both vertically and horizontally. Owing to limitations with Matlab, this objective was not perfectly realized.

All of these JPEG profiles (both the EarthImager 2D and Matlab versions) are available in this publication as well as the raw and processed data files. These data are available from the GIS/Data Catalog page, with previews of the profiles available from the Preview pages.

Matlab was also used to remove that portion of the resistivity data from the XYZ files that fell within the water column, based on the DEP data files. These files are the "trimxyz" files. Both the DEP and XYZ data were interpolated within Matlab so that a resistivity value could be extracted for the sediment/water interface. That interpolated value, along with the measured values in the sediment, was exported to this revised XYZ data file. Data processed in this way could also be compared with the related geophysical data, such as seismic-reflection data, which have been collected previously in many coastal areas (Cross et al., 2005; Poppe et al., 2006).

Finally, a Visual Basic 6 program was written to combine the linearized STG file with the DEP file from each survey line to create a data file in RES2DINV format for users of that software package. These files are also available from the GIS/Data Catalog page.