USGS Groundwater Information: Branch of Geophysics
The USGS Connecticut Water Science Center is currently conducting an investigation of nitrogen discharge from surficial and bedrock aquifers to the Niantic River estuary in Niantic, Connecticut. The goal of the project is to provide information that will assist in setting nitrogen criteria for eelgrass beds. Eelgrass beds have been in decline in Long Island Sound, and it is suspected that ground-water discharge of nitrogen is one of the causes. USGS hydrologist John Mullaney worked with the USGS Office of Ground Water, Branch of Geophysics (OGW BG) to conduct geophysical investigations designed to delineate the spatial distribution of freshwater and possible locations of focused ground-water discharge to the Niantic River. Mullaney and OGW BG hydrologist Eric White collected over 4.5 kilometers (km) of continuous-resistivity profile (CRP) data.
OGW BG includes an 11-electrode streamer with either 5- or 10-meter (m) spacing between electrodes. As with land-based resistivity methods, increasing the separation between the current and potential electrodes increases the depth of investigation. According to White, typical depths of investigation are about 10 m for the streamer with a 5-m electrode spacing, or 20 m for the 10-m streamer. Data are collected about once per meter as the boat (and streamer) move through the water.
"Collection of GPS data is crucial," explains White, "both for transforming the location of the data points from local coordinate grid to a geographic or projected system, and also for the data analysis, which requires spatially referenced measurements."
In addition to collecting GPS data, White also acquires water-column data: "We collect single-beam bathymetry data from the boat, as we go. Also, we use a Hydrolab or similar probe to measure the water-column specific conductance." Recent studies by OGW BG (Loke and Lane, 2004; Lane and others, 2005) have shown that incorporating bathymetry and water-column resistivity information in resistivity data inversions leads to more reliable and higher-resolution results.
Analysis of CRP data produces cross-sectional images (tomograms) of the subsurface resistivity structure. White says, "We can collect a lot of data in very little time, up to perhaps 20 km per day in ideal conditions. Then we need about 3 days of data processing for each day spent acquiring data." The Niantic project tomograms processed by White extend to depths of about 22 m and show areas of submarine freshwater.
According to Mullaney, "The CRP data are helpful, in conjunction with conventional test drilling and ground-water quality sampling, to understand the mechanism of ground-water discharge to the estuary -- a concept that is poorly understood." He adds, "It has been great working with OGW BG because of their enthusiasm for furthering the use of geophysics in USGS State Water Science Center projects."
OGW BG has used CRP in other investigations in support of WSC activities around the country. The USGS Nebraska Water Science Center used CRP to define a scour zone and estimate the volume of sediment removed by river flooding. For a project in North Carolina, CRP was used to identify magnetic dikes underlying a major river as well as to delineate a section of gaining reach for a nitrate contamination study: the CRP interpretations are being used to guide further work including river discharge measurements and streambed-piezometer specific-conductance profiles.
According to John W. Lane, Jr., the OGW BG Chief, "We're seeing more and more interest in CRP. CRP can provide information about river, lake, and shallow ocean subbottom materials and reveal locations of focused ground-water discharge important for understanding ground-water/surface-water interaction at scales relevant to many water studies. The instrumentation and processing software have come a long way in the last few years, which facilitates training and tech transfer for those interested in including CRP in their studies."
For more information about CRP, visit http://water.usgs.gov/ogw/bgas/crp/.
Loke, M.H., and Lane, J.W., Jr., 2004, Inversion of data from electrical resistivity imaging surveys in water-covered areas: Exploration Geophysics, v. 35, p. 266-271.
Lane, J.W., Jr., F.D. Day-Lewis, M.H. Loke, and Eric A. White, E.A., 2005, Pitfalls in inversion and interpretation of continuous resistivity profiling data-effects of resolution limitations and measurement error [abs.]: EOS Transactions, American Geophysical Union, Fall Meeting Supplement, v. 86, no. 52, abstract H43F-0546.
Do you have a project you would like to see highlighted in our next Profiles in Geophysics? Contact OGW BG at GS-W_OGW_BG_info@usgs.gov.