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Application of geophysical methods to the delineation of paleochannels and missing confining units above the Castle Hayne aquifer at U.S. Marine Corps Air Station, Cherry Point, North Carolina
The U.S. Marine Corps Air Station, Cherry Point, North Carolina, is underlain by four freshwater-bearing aquifers--the surficial, Yorktown, and upper and lower Castle Hayne. The upper and lower Castle Hayne aquifers serve as the principal supply of freshwater for the Air Station. The potential for movement of contaminated water from the surficial aquifer downward to the water-supply aquifer is greatest in areas where clay confining units are missing. Missing confining units may indicate the presence of paleochannels filled with permeable material. Seismic-reflection techniques were successful in delinea- ting paleochannels of Quaternary and Tertiary age within unconsoli- dated sediments less than 180 feet deep at several locations. Continuous single-channel marine seismic-reflection profiling in the Neuse River was effective in delineating a large paleochannel complex consisting of at least two superimposed paleochannels within hydrogeologic units overlying the upper Castle Hayne aquifer. The complex was found immediately north of the Air Station and is thought to continue south beneath the Air Station. Shallow high-resolution land seismic-reflection techniques were used at the Air Station to delineate structures and correlate strati- graphy between the limestone of the upper Castle Hayne aquifer and the Yorktown confining unit. Three different land seismic-reflection techniques proved effective for the horizontal extrapolation of geo- logic features and identification of paleochannels at several locations. The northeastern margin of a large paleochannel was identified beneath the southern part of the Air Station. This feature strikes northwest to southeast and cuts through the Yorktown and upper Castle Hayne aquifer confining units.
Use of geophysical logs to estimate the quality of ground water and the permeability of aquifers
The relation of formation factor to resistivity of formation water and intergranular permeability has often been investigated, and the general consensus is that this relation is closest when established in a clean-sand aquifer in which water quality does not vary substantially. When these restrictions are applied, the following standard equation is a useful tool in estimating the resistance of the formation water: F = Ro/Rw, where F is the formation factor, which is a function of the effective porosity; Ro is the resistivity of a formation that is 100 percent saturated with interstitial water; and Rw is the resistivity of the water in the saturated zone. However, arenaceous aquifers can have electrical resistivities that are not directly related to resistivity of water or porosity. Surface conductivity and ion exchange are significant factors when the sediments are clay bearing. The solid constituents are a major component of the parameters needed to solve the equation for formation-water resistivity and estimates of aquifer permeability. A correction process needs to be applied to adjust the variables, Ro and F, to the equivalent of clean sand. This report presents an empirical method of using the neutron log and the electrical-resistivity values from long- and short-normal resistivity logs to correct for fine-grained material and the subsequent effects of low impedance to electrical flow that are not related to the resistance of formation water.
Geological and geophysical study of the preglacial Teays Valley in west-central Ohio
No abstract available.
Borehole geophysical methods for analyzing specific capacity of multiaquifer wells : ground-water hydraulics
Conventional well-logging techniques, combined with measurements of flow velocity in the borehole, can provide information on the discharge-drawdown characteriBtic8 of the several aquifers penetrated by a well. The information is most conveniently presented in a graph showing aquifer discharges as functions of the water level in the well at a particular time. To determine the discharge-drawdown characteristics, a well is pumped at a steady rate for a certain length of time. While the well is being pumped, measurements are made of drawdown and of the discharge rates of the individual aquifers within the well. Discharge rates and drawdowns ,are usually recorded as functions of time, and their values for any given time during the test are obtained by interpolation. The procedure is repeated for several different rates of total well discharge. The well may be allowed to recover after each step, or discharge may be changed from one rate to another, and changes in discharge and drawdown may be measured by extrapolation. The flow measurements within the well may be made by use of a subsurface flowmeter or by one of several techniques involving the injection of electrolytic or radioactive tracers. The method was tested on a well in Mercer County, Pa., and provided much useful information on aquifer yields, 'thieving,' and hydrostatic heads of the individual zones.
Using geophysical logs to estimate porosity, water resistivity, and intrinsic permeability
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Structural evolution of the east Sierra Valley system (Owens Valley and vicinity), California: a geologic and geophysical synthesis
The tectonically active East Sierra Valley System (ESVS), which comprises the westernmost part of the Walker Lane-Eastern California Shear Zone, marks the boundary between the highly extended Basin and Range Province and the largely coherent Sierra Nevada-Great Valley microplate (SN-GVm), which is moving relatively NW. The recent history of the ESVS is characterized by oblique extension partitioned between NNW-striking normal and strike-slip faults oriented at an angle to the more northwesterly relative motion of the SN-GVm. Spatially variable extension and right-lateral shear have resulted in a longitudinally segmented valley system composed of diverse geomorphic and structural elements, including a discontinuous series of deep basins detected through analysis of isostatic gravity anomalies. Extension in the ESVS probably began in the middle Miocene in response to initial westward movement of the SN-GVm relative to the Colorado Plateau. At ca. 3-3.5 Ma, the SN-GVm became structurally separated from blocks directly to the east, resulting in significant basin-forming deformation in the ESVS. We propose a structural model that links high-angle normal faulting in the ESVS with coeval low-angle detachment faulting in adjacent areas to the east.
A new analysis of caldera unrest through the integration of geophysical data and FEM modeling: The Long Valley caldera case study
The Long Valley Caldera, located at the eastern edge of the Sierra Nevada range in California, has been in a state of unrest since the late 1970s. Seismic, gravity and geodetic data strongly suggest that the source of unrest is an intrusion beneath the caldera resurgent dome. However, it is not clear yet if the main contribution to the deformation comes from pulses of ascending high-pressure hydrothermal fluids or low viscosity magmatic melts. To characterize the nature of the intrusion, we developed a 3D finite element model which includes topography and crust heterogeneities. We first performed joint numerical inversions of uplift and Electronic Distance Measurement baseline length change data, collected during the period 1985–1999, to infer the deformation-source size, position, and overpressure. Successively, we used this information to refine the source overpressure estimation, compute the gravity potential and infer the intrusion density from the inversion of deformation and gravity data collected in 1982–1998. The deformation source is located beneath the resurgent dome, at a depth of 7.5 ± 0.5 km and a volume change of 0.21 ± 0.04 km 3 . We assumed a rhyolite compressibility of 0.026 ± 0.0011 GPa −1 (volume fraction of water between 0% and 30%) and estimated a reservoir compressibility of 0.147 ± 0.037 GPa −1 . We obtained a density of 1856 ± 72 kg/m 3 . This density is consistent with a rhyolite melt, with 20% to 30% of dissolved hydrothermal fluids.
Neotectonics of the eastern Juan de Fuca Strait: A digital geological and geophysical atlas
No abstract available.
Geophysical characterization of a fractured-bedrock aquifer and blast-fractured contaminant-recovery trench
No abstract available.
Electrical geophysical study over the Norman Landfill, near Norman, Oklahoma
No abstract available.
An integrated geophysical approach for groundwater and seismic hazard management in Joshua Tree National Park, southern California
Two‐dimensional inversion of audiomagnetotelluric (AMT) sounding data define buried resistivity distributions that reflect subsurface geology and structure within the upper kilometer beneath Pleasant Valley, a 1–2 km‐deep pull‐apart basin in Joshua Tree National Park, southern California. The Park lies within the Eastern California Shear Zone just east of the San Andreas Fault, and is surrounded by developing desert communities. Understanding the subsurface in and around the Park is important for management of groundwater resources, for mitigation of seismic hazards, and for unraveling the tectonic evolution of the region. Our resistivity models, interpreted in conjunction with gravity inversions, show transitions between coarse‐grained and fine‐grained alluvium, resistive (> 400 ohm‐m) crystalline rocks, and the locations of range‐front and intra‐basin faults.
Geochemistry and geophysics of iron oxide-apatite deposits and associated waste piles with implications for potential rare earth element resources from ore and historic mine waste in the eastern Adirondack Highlands, New York, USA
The iron oxide-apatite (IOA) deposits of the eastern Adirondack Highlands, New York, are historical high-grade magnetite mines that contain variable concentrations of rare earth element (REE)-bearing apatite crystals. The majority of the deposits are hosted within sodically altered Lyon Mountain granite gneiss, although some deposits occur within paragneiss, gabbro, anorthosite, or potassically altered Lyon Mountain granite gneiss. The IOA deposits and the waste and/or tailings piles associated with them have potential as an unconventional resource for REEs. Reprocessing of these piles would have the advantage of partial recycling of the waste material to produce a set of critical elements. Thirty-four ore, nine rock, 25 waste-pile, and four tailings-pile samples were collected and analyzed for major, minor, and trace elements. At the tailings- and waste-pile sites, composite samples were collected by combining 30 to >50 subsamples randomly distributed over each pile. The total REE content of the waste and tailings piles varied from approximately 10 to 22,000 ppm, whereas the ore sample concentrations ranged from approximately 15 to 48,000 ppm total REEs. A positive correlation exists between the total REE content of ore and its associated waste pile. Median light REE/heavy REE values were 2.14 for waste/tailings piles and 2.25 for ore, which is a substantial relative enrichment in the heavy REEs in comparison to many developed REE mines, such as the mined carbonatites of Bayan Obo, China, and Mountain Pass, California. Importantly, the ore and waste samples are significantly enriched in both Y and Nd compared to other REEs in the samples. Other minor components such as Th are also elevated. Airborne radiometric surveys show large positive eTh and eU anomalies corresponding to tailings piles. Although it is a limited data set, geochemical data of unaltered and altered host rocks suggest a speculative new model for IOA ore formation in the Adirondack Highlands that is consistent with the geology and previously published data. The ferroan ore-hosting Lyon Mountain granite gneiss underwent localized potassic alteration that enriched the altered rock in Fe, REEs, Th, and other metals. A later sodic alteration event affected the previously potassically altered Lyon Mountain granite gneiss, which increased rock porosity and remobilized Fe, REEs, and other elements from the host rock into the iron ore seams. The sodic fluids responsible for ore formation were enriched in F and Cl.
Geophysical observations of Kilauea volcano, Hawaii, 1. temporal gravity variations related to the 29 November, 1975, M = 7.2 earthquake and associated summit collapse
Repeated high-precision gravity measurements made near the summit of Kilauea volcano, Hawaii, have revealed systematic temporal variations in the gravity field associated with a major deflation of the volcano that followed the 29 November, 1975, earthquake and eruption. Changes in the gravity field with respect to a stable reference station on the south flank of neighboring Mauna Loa volcano were measured at 18 sites in the summit region of Kilauea and at 4 sites far removed from its summit. The original survey, conducted 10-23 November, 1975, was repeated during a two-week period after the earthquake. The results indicate that sometime between the first survey and the latter part of the second survey the gravity field at sites near the summit increased with respect to that at sites far removed from the summit. The pattern of gravity increase is essentially radially symmetrical, with a half-width slightly less than 3 km, about the point of maximum change 1.5 km southeast of Halemaumau pit crater. Gravity changes at sites near the summit correlate closely with elevation decreases that occurred sometime between leveling surveys conducted in late September 1975 and early January 1976. The systematic relation between gravity and elevation change (-1.71 ?? 0.05 (s.e.) ??gal/cm) shows that deflation was accompanied by a loss of mass from beneath the summit region. Mass balance calculations indicate that for all reasonable magma densities, the volume of magma withdrawn from beneath the summit region exceeded the volume of summit collapse. Analysis suggests that magma drained from at least two distinct areas south of Kilauea caldera that coincide roughly with two reservoir areas active during inflation before the 1967-1968 Kilauea eruption. ?? 1980.
SURVEYS OF COASTAL STRUCTURES USING GEOPHYSICAL TECHNIQUES.
During the summer of 1983 and the spring and summer of 1984, the authors conducted side scan sonar and shallow subbottom surveys in conjunction with bathymetric and diving surveys along three northern California coastal structures to determine the condition of the structures before extensive damage occurred. Then the applicability of the data collection techniques for condition surveys in general was evaluated. Two of the structures surveyed are the parallel jetties that protect the entrance to Humboldt Bay, California, and the third structure is the outer breakwater at Crescent City, California. It was found that the side scan sonar is an excellent tool for defining the toe of the structure; also, when waves are low, it can be useful in determining armor types and the slope of structures. The subbottom system provided information on fault and bedrock locations; however, other systems need to be tested to see if any of them can locate buried armor near the structures.
Conference on "Isotope Tracers in Geochemistry and Geophysics". In honor of Professor Gerald J. Wasserburg on the occasion of his sixtieth birthday
[No abstract available]