This data release consists of three different types of data: including direct current (DC) resistivity profiles, frequency domain electromagnetic (FDEM) survey data, and global navigation satellite system (GNSS) coordinate data of the geophysical measurement locations. A data dictionary is included along with the data and defines all of the table headings, definitions, and units. Earthen dams are common on lakes and ponds, but characteristics of these structures such as construction history, composition, and integrity are often unknown for older dams. Geophysical surveying techniques provide a non-invasive method of mapping their lithology and structure. In particular, DC resistivity and FDEM methods can, when properly processed, provide the information necessary to construct a lithologic model of an earthen dam without having to trench or core through the shell of the dam itself. In September, 2016 the U.S. Geological Survey (USGS) conducted geophysical surveys at Bob Kidd Lake, an 81-hectare lake, in northwestern Arkansas to help determine the composition of the earthen dam and guide any potential geotechnical investigations. A series of DC resistivity surveys were conducted along, parallel, and perpendicular to the axis of the crest of the dam to identify the soil-bedrock interface and any variations in the composition of the earthen dam. A dense survey using a multi-frequency electromagnetic sensor was used to map the shallow materials comprising the dam at a higher resolution. Resistivity measurements were made by transmitting a known current through two electrodes (transmitter) and measuring the voltage potential across two other electrodes (receiver). The multiple channels on the resistivity meter allow for voltage measurements to be made at 10 receivers simultaneously following a current injection. The configuration of the transmitter relative to the receiver(s) is referred to as an array. For these surveys, a Reciprocal Schlumberger array was used, which positions the transmitting pair of electrodes toward the center of the array and the receiving pairs extending away from the transmitter (Loke, 2000; Zonge and others, 2005). The electrical resistance was calculated by dividing the measured voltage by the applied current. The apparent resistivity was determined by multiplying the electrical resistance by a geometric factor. Apparent resistivity is not the true resistivity, but rather a volume-averaged estimate of the true resistivity distribution, because a homogeneous, isotropic subsurface is assumed. To estimate the true resistivity of the heterogeneous and/or anisotropic subsurface, the apparent resistivity data were processed using an inverse modeling software program. The FDEM method complements the two-dimensional (2-D) DC resistivity method and was used to extend the depth of subsurface characterization obtained with resistivity profiles. The FDEM method uses multiple current frequencies to measure bulk electric conductivity values (the inverse of resistivity values) of the earth at different depths (Lucius and others, 2007). For this project FDEM data were collected with a GEM-2, a broadband, multifrequency, fixed-coil electromagnetic induction unit (Geophex, 2015). In addition to the geophysical surveys a concurrent Global Navigation Satellite System (GNSS) survey was conducted using a Real Time Kinematic system (RTK). All electrode locations on the DC resistivity profiles, all measurement locations in the FDEM survey, as well as a point-cloud survey were collected and are included in the dataset. These data were used to geo-reference the geophysical data and may be used to create a Digital Elevation Model (DEM) of the dam surface.
Geophysical surveys and geospatial data for Bob Kidd Lake, Washington County, Arkansas