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Continuous Resistivity Profiling

OGW BG collaborated with Advanced Geosciences, Inc. (AGI) to test the new OGW BG continuous resistivity profiling (CRP) system. AGI provided training on CRP data acquisition, processing and interpretation from January 7 to January 11, 2002, for John Lane and Eric White of the USGS OGW, Branch of Geophysics and Marcel Belaval a graduate student at Boston College. The training was conducted at AGI and on-site at Lake Travis, Austin, Texas.

What is CRP?

CRP is an emerging technology that is used to image the subsurface electrical properties of an estuarine, riverine or lacustrine environments.  Apparent resistivity data are collected and inverted to determine the true subsurface resistivity distribution.  Subsurface structure and geology can then be interpreted from the resistivity information. Continuous operation of the CRP system allows for rapid data collection (10 to 20 kilometers/day).

Why collect CRP data?

CRP could be used for:

• mapping fresh water seeps,
• imaging the saltwater/freshwater interface,
• investigating salt-water marshes and coastal embayments,
• developing a hydrogeologic framework,
• monitoring dam removal,
• delineating of sediment types, and
• mapping sub-bottom contamination plumes.

CRP data complement other water-borne geophysical tools, such as continuous seismic profiling (CSP) and waterborne radar.

Description of OGW BG CRP System

The new CRP system, available for USGS use, consists of an 8-channel resistivity system and an electrode streamer with 11 electrodes at a 10-meter (m) spacing. Four variations of the dipole-dipole array can be used. The system is designed for sub-bottom resistivity imaging in shallow water (<20 m, fresh water) environments.

Sample CRP Data

During the survey on Lake Travis, four array types were tested along a single survey track approximately 2.5 kilometers long. Depth of penetration ranged from 15 to 20 m in fresh water. Resistive anomalies (warmer colors in the image below) in the inverted resistivity model correspond to physical water-bottom.

Off-the-shelf software can be used to invert the resistivity data. Constraining the depth and specific conductance of the water-column can reduce surface noise caused by artifacts of the inversion process. These constraints are particularly useful for areas of decreased resistivity.

The profiles below were collected along the Thames River, Norwich, Connecticut. Cooler colors represent regions of lower resistivity, and warmer colors regions of higher resistivity. The top profile is the measured apparent resistivity pseudosection. The middle profile is the inverted resistivity model with no constraints on water depth or specific conductance. The bottom profile is the inverted resistivity model with constraints on water depth and specific conductance.

USGS offices and cooperators can contact John Lane (Chief, OGW BG) at jwlane@usgs.gov or (860) 487-7402 x13 to learn more about the CRP system or to discuss related training needs.

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