Click on figure for larger image.

Figure 4. Continuous resistivity profiling (CRP) streamer deployed behind the survey boat (photo is from a different survey). Foam flotation is attached to the cable between each electrode. Photo by John Bratton, NOAA.

Figure 5. Continuous resistivity profiling (CRP) data acquisition system in the survey boat (photo is from a different survey). The yellow box to the left is the AGI SuperSting R8 resistivity meter, the black box in the middle manages the battery power to the system, and the yellow box to the right contains the Lowrance GPS-enabled fathometer. Photo by John Bratton, NOAA.

Data collection and processing methods are described here for continuous resistivity profiling (CRP) data collected aboard the R/V Terrapin from May 12 to May 17, 2008 (Julian day 133 to Julian day 138), in two locations on the north shore of Long Island. Data collected May 12 to May 14 were collected in Northport Harbor, the more eastern of the two collection sites. Data collected May 15 to May 17 were collected in Manhasset Bay in western Long Island.

Continuous Resistivity Profiling (CRP)

CRP data were collected in Northport Harbor and Manhasset Bay in 2008 using methods similar to those described by Cross and others (2008, 2010). In both locations, data were collected with a 50-m streamer and a 15-m streamer on separate days. The table below summarizes the data collection.

Data were collected using an Advanced Geosciences, Inc. (AGI) system (figs. 4 and 5) with two separate resistivity streamers. The AGI 50-m streamer is an 11-electrode array with electrodes spaced 5 m apart. The two source electrodes are graphite, while the nine receiver electrodes are stainless steel. The AGI 15-m streamer is an 11-electrode array with electrodes spaced 1.5 meters apart. As with the 50-m streamer, the two source electrodes are graphite, while the nine receiver electrodes are stainless steel. In both cases, foam flotation is attached to the cable between each electrode to keep the cable at or near the surface while allowing the electrodes to hang a little below the water surface and helping to keep the electrodes in the water in mildly choppy seas. The depth of penetration of these systems is approximately 25 percent of the towed cable length. 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 nine electrodes.

Among other applications, CRP surveys are conducted to determine electrical resistivity of the subsurface in order to distinguish fresh from saline groundwater in shallow sedimentary units over large areas (Manheim and others, 2004; Cross and others, 2008, 2010). The initial values recorded by the system are measured apparent resistivity values. A two-dimensional (2D) resistivity model takes into account resistivity changes in the vertical and horizontal directions along a survey line, while assuming resistivity remains constant perpendicular to the survey line (Loke, 2000). The apparent resistivity data undergo an inversion process, which produces a resistivity profile that is most consistent with the measured values. EarthImager 2D inversion software divides the subsurface into a number of rectangular blocks. The resistivity of each 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. The data, both raw and processed, including metadata compliant with Federal Geographic Data Committee (FGDC) standards, can be accessed through the Data Catalog page.

The shipboard control and logging system used for the data collection of the resistivity streamer was an AGI SuperSting R8 Marine resistivity meter (fig. 5). Current was injected into the water/sediment system approximately every 3 seconds through the source electrode pair at the front end of the streamer, and eight apparent resistivity values representing eight depth levels were recorded for each current injection. A Lowrance LMS-480M GPS with an LGD-2000 GPS antenna and a 200-kilohertz fathometer transducer with a temperature sensor were also attached to the system in order to acquire navigation and depth.

At the end of each day of data acquisition, the CRP and navigation/depth data were downloaded from the SuperSting system using a laptop running AGI's Marine Log Manager software. The data were then transferred to a processing computer in order to check the quality of the navigation data and process the CRP data using AGI's Marine Log Manager and EarthImager 2D software packages. The Marine Log Manager software was used to merge the navigation file (file extension GPS) with the raw resistivity data (file extension STG), resulting in a linearized resistivity file (STG file extension) and a file containing the depth and temperature data when available (DEP file extension). These files are used as input to AGI's EarthImager 2D software for data processing.

The DEP files can be modified to include the water resistivity value measured at the time of surveying. For this survey, 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 23 in Northport Harbor and 22.3 in Manhasset Bay. The average temperature in Northport Harbor was 15.6 degrees Celsius, while Manhasset Bay measured 15 degrees Celsius. Based on these values, the calculated water resistivity value during the survey in Northport Harbor was 0.34 ohm-meters (ohm-m) and 0.35 ohm-m in Manhasset Bay. These values were added to the appropriate DEP files used in processing all of the data.

EarthImager 2D version 2.2.8 was used to process the data files (STG and DEP) using the supplied CRP saltwater processing parameters, which have minimum and maximum allowable resistance and voltages appropriate for the marine environment. The EarthImager 2D CRP module is specifically designed to process large amounts of continuous resistivity data, as are typically acquired during marine surveys. The strategy for data 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). The output files from all of these steps are saved into an individual folder. For the purposes of this report, the linearized STG and DEP files used as input for the processing were saved, as well as three file types generated during processing. These include:

1. JPEG images of the complete resistivity profile. Both long (uncompressed, same horizontal scale) and short (compressed to fit on a single page, therefore having different horizontal scales) versions are available for each line.
2. Text files of the processed resistivity data. These include XYZ files in which X = distance along line, Y = depth, and Z = resistivity value. Also included are LLT files, which contain longitude, latitude, depth, and resistivity values.
3. A file with the INI extension 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 the processing parameters used.

The JPEG images resulting from the EarthImager 2D processing were saved with the default color scale. This color scale ranges from blues to reds, with reds representing the higher resistivity values, corresponding to fresher (less saline) groundwater. The color scale within each image is maximized for the range of resistivity values from that survey line.

In addition, in order to more easily compare different resistivity profiles, MATLAB software was used to combine the XYZ and DEP files to generate JPEG images with a common color scale for all survey line files. Within 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 reds indicating higher resistivity values. MATLAB software was also used to plot the data in an attempt to display the JPEG images with a common vertical and horizontal distance scale. Rounding errors in figure size scaling prevent exact reproducibility of scale in the horizontal direction for the images.

Both the EarthImager 2D and the MATLAB JPEG images can be accessed from the Profile Previews pages.

MATLAB was also used to remove the water-column resistivity data from the XYZ files based on the water depth data in the DEP data files. Both the DEP and XYZ data were interpolated within MATLAB in order to extract a resistivity value for the sediment/water interface. The interpolated value, along with the measured values within the sediment, was exported to the modified XYZ data file. All of the CRP lines were processed, and the results were combined into a single shapefile based on survey area (northportall_resbelowsed and manhassetall_resbelowsed) available from the Data Catalog page.

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 geoelectrical imaging software package. These files are available from the Data Catalog page.