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14th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 04 Mar 2001 - 07 Mar 2001
- Location: Denver, Colorado, USA
- Published: 04 March 2001
61 - 80 of 129 results
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Sonic Pulse Velocity Testing To Assess Condition Of A Concrete Dam
Authors Edward D. Billington, Dennis A. Sack and Larry D. OlsonA velocity analysis was performed to assess the condition of a concrete arch dam. Sonic pulse
velocity (SPV) measurements were made at 28 locations along the dam by impacting the downstream
face and recording arrival times on the upstream face. The sonic energy was generating using a remotely
operated impact source specifically designed for the project that could be lowered from the dam crest.
The compressional wave energy was recorded using a string of hydrophones on the upstream face of the
dam. A tomographic analysis of the arrival time data was performed to generate cross sections at each
station to show the velocity distribution through the dam. An underwater inspection of the upstream face
provided the opportunity to correlate velocity anomalies with diver observations. The results of this
study will serve as a baseline for future assessments.
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High Resolution Seismic Reflection Survey At Keechelus Dam
Authors Richard Miller and Richard MarkiewiczReplacement of the existing Bureau of Reclamation dam (Keechelus) located at the headwaters
of the Yakima River in Washington became necessary after the incidental encounter of a void during
trenching along the dam crest. Subsequent geophysical surveys discovered several other voids along the
structure, mandating the replacement of this high-risk dam. High resolution seismic reflection surveys
conducted along the downstream toe provided a continuous image of till, swamp bog, alluvial, outwash,
and lacustrine sediments which make up the Quaternary unconsolidated material that lay on Tertiary
rhyolite bedrock. Each of the three seismic lines possessed uniquely different signal-to-noise and resolution
characteristics. One of the profiles was acquired perpendicular to the long axis of the dam and
through a swamp area. This profile provided an excellent image of the till/lacustrine sediment contact
with indications of very steeply dipping contacts between the lacustrine and outwash sediments. Delineation
of these features, as shallow as 20 ft, is significant to the engineering of a new structure, which
must provide a watertight seal through the very permeable outwash and alluvial fan sediments. Resolution
potential of this 24-fold, 150+ Hz data set is around 10 ft (~½ wavelength). A dramatic channel type
feature separating a thin sequence of outwash and lacustrine sediments from a thickening wedge of
lacustrine type sediments is interpretable directly beneath the trace of the proposed new structure. Based
on the seismic reflection data set it should be possible to more precisely fit the preconstruction engineering
designs to the actual geology, which will be encountered during construction.
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The Use Of Non-Invasive Geophysics To Assess Damage By Burrowing Animals To Earthen Levees Near Dayton, Ohio
Authors Matthew Barner, Ernest Hauser and Paul WolfeGround-penetrating radar (GPR), electrical resistivity, and streaming potential methods were
employed at southwestern Ohio, flood-control levee sites to test their effectiveness for determining the
amount of damage by animal burrows. Three 36-m long sections of earthen levee were tested, each
containing visible groundhog burrows. Overall levee dimensions and burrow locations dictated the
extent of the field surveys at each site. The GPR data were gathered with 80 and 300 MHz antennas.
2D electrical resistivity imaging was performed using a system with a dipole-dipole electrode
configuration. Electrical resistivity data were collected under dry conditions and while saline water was
pumped into selected visible burrows. The intent was to increase the resistivity contrast of the burrows.
Non-polarizing electrodes were used to gather streaming potential data. One core sample taken at each
site helped provide information on levee composition.
The GPR data provided the most reliable information on levee damage at the West Carrollton
site. Depth of penetration and resolution were good enough to map potential burrows to depths of about
2.4 meters and show a potentially interconnected network of burrows throughout the upper half of the
levee. GPR data for the other levees yielded results only to depths of around 1.5 meters and with lower
resolution compared to the West Carrollton data. Electrical resistivity imaging obtained adequate depth
penetration at all three sites, although resolution was too poor to effectively mark any burrows and
estimate any internal damage caused by the burrows. The streaming potential data lacked signal above
the noise range and thus provided little useful information. Based on the GPR data, significant
groundhog damage may exist in the West Carrollton levee. In addition, this levee may contain an
extensive, interconnected network of burrows. Not enough information is available for the Dayton and
Middletown sites to make similar conclusions.
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Dependence Of Sandstone Dielectric Behaviour On Moisture Content And Lithology
Authors L.J. West, Y. Huang and K. HandleyAn investigation of the radar frequency dielectric behaviour of Sherwood Sandstone from
Yorkshire, England is reported. The equipment used, a large one port co-axial cell, and the method of
data interpretation are described. The co-axial cell measures the frequency dependent complex
reflection co-efficient of drill core samples. The dielectric properties are extracted from the complex
reflection co-efficient, the cell calibration data, and specimen geometry. Proof testing of the dielectric
cell is described. The cell works well over the frequency range of 75 to 1000MHz for saturated
specimens, and over the frequency range of 200 to 1000MHz for dry specimens.
The frequency dependent dielectric behaviour of sandstone specimens with mean grainsize
ranging from 300 to 125 microns over the full range of saturation levels is described (samples were
progressively wetted by addition of de-ionised water, saturated by soaking, and then dried by
evaporation). The results of the study show that the real component of the dielectric constant, Kr, is
independent of frequency and lithology for between 350MHz and 1000MHz. Below 350MHz, dielectric
behaviour is lithology dependent, with finer grained specimens showing higher Kr than medium grained
specimens.
The behaviour of Kr with moisture content is compared with that predicted by the Topp equation
and the Complex Refractive Index Method (CRIM). The Topp equation, and CRIM with Ks between
3.5 and 5 provide a good match to the data for all lithologies at 500MHz. They are also likely to provide
a good match at 100MHz for the medium grained sandstone lithology. The results indicate that
determination of moisture content/porosity is best done at higher frequencies (e.g. using Time Domain
Reflectometry) where a unique relationship exists. In contrast, characterisation of lithology from
dielectric behaviour is best carried out at lower frequencies (i.e. in 50 or 100MHz radar surveys), where
the dielectric constant is lithology dependent.
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Small-Strain Elastic Properties Of Compacted Clayey Soils During Drying
Authors Gokhan Inci, Takaaki Kagawa and Nazli YesillerIn this study ultrasonic methods were used for the determination of small-strain elastic properties
of compacted clayey soils. Three clayey soils compacted at different moisture contents and compactive
efforts were tested while drying. Piezoelectric transducers were used to generate and receive P- and Swaves
on cubical soil samples with oblique corner cuts. Five elastic parameters of cross-anisotropy
were calculated using three P-wave and two S-wave velocity measurements. Constrained, Young's, and
shear moduli in the vertical and horizontal directions increased during drying. Young's and shear
moduli increased monotonically, while constrained modulus exhibited a peak for wet of optimum and
optimum water content samples. It was observed that Poisson's ratios decreased during drying and that
the soils compacted with high compactive effort had lower Poisson's ratios compared to the soils
compacted with low compactive effort at the same saturation level. Anisotropy value, n, increased for
wet of optimum and optimum water content samples during drying, which was attributed to particle
reorientation during shrinkage process. On the other hand, n did not change significantly for soils
compacted at dry of optimum water content.
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Mapping Karst Conditions Using 2D And 3D Resistivity Imaging Methods
Authors Tomasz Z. Labuda and Andrew C BaxterAuthors of this document have performed several geophysical investigations within the eastern
United States. Most of this experience has involved subsurface investigations in carbonate rock units.
As compared with different geophysical techniques including micro gravity, electromagnetics, ground
penetrating radar, spontaneous potential, and other methods, the best results in characterizing subsurface
conditions in karst terrain were accomplished using surface resistivity imaging.
The following discussion is based on two case studies, both involving two-dimensional and theedimensional
surface resistivity surveys performed over Paleozoic age carbonate rocks in northern
Virginia and West Virginia. The purpose of this document is to show the benefits and pitfalls of twodimensional
and three-dimensional resistivity imaging used to characterize subsurface conditions in the
karst terrain. Both methods are compared based on different site geologies, array geometries, and target
size and depth. A short discussion of dipole-dipole method commonly used in two-dimensional arrays,
versus pole-pole used in three-dimensional arrays is presented to indicate differences in expected data
resolution.
The intention of this document was also to highlight the importance of volumetric visualization
of three-dimensional data sets for analysis and interpretations. Creating volumetric three-dimensional
models of resistivity is useful in evaluating the location, orientation and character of potential fracture
systems within relatively uniform rock masses.
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Seismic Characterization Of The Karst Bedrock Surface In The Southeast Industrial Area At Anniston Army Depot, Alabama
Authors M.D. Thompson, S.F. Miller, M.A. Glennon, J.M. Cooper and W.A. MandellEnvironmental concerns at the Southeast Industrial Area (SIA) of Anniston Army Depot
(ANAD) have been the impetus for geotechnical investigations over the last 20 years. These previous
investigations have all failed in one respect, to map the bedrock topography of the underlying Knox
Group dolomites. The bedrock surface is an important target: a) lows points provide a greater column
of overburden in which to trap contaminants; b) bedrock pinnacles may act as rapid access points into
the karst conduit systems; and c) hydraulically transmissive zones are often found within the weathered
portion (epikarst) of the bedrock. The approach taken by Argonne was to construct a bedrock surface
map integrating existing borehole data with selective seismic-refraction profiling. Initial inspection
revealed that depth-to-bedrock measurements based solely on auger refusal were not acceptable in
many cases because refusal occurred on top of either large boulders or chert horizons.
Seismic refraction profiling within the SIA required overcoming several technical challenges.
First, numerous sumps, fans, and blowers produced strong background noise, as did the constant
vehicular traffic and sporadic railroad activity. Secondly, construction design for the SIA was not
conducive to seismic profiling as compacted gravel-fill, reinforced concrete, and asphalt paving
comprised the ground surface. One solution was simple brute force with upwards of 30 stacks using a
weight-drop source to overcome the background din. Other solutions included collecting data during
off-hour periods, mounting geophones in gravel, grassy, and asphalt areas adjacent to the major
roadways, and paying careful attention to where the shot point was located to avoid impacting on the
hardened concrete surfaces. In the latter case, the concrete roadbed acts as a wave-guide, primarily
transmitting the seismic energy horizontally and obscuring the underlying geology.
Seismic profiling and analysis of borehole data indicate that at least 4 major bedrock
depressions, averaging 20-to-30 m in local relief, occur along the southeast and northeast boundaries.
Drilling records indicate more frequent occurrences of clay-filled cavities and fractured and weathered
rock along the southeast boundary, suggesting a causal relationship for the location of the bedrock
lows. Eight confirmatory boreholes were used to ground truth the seismic models. Four of these
boreholes confirmed the presence of the bedrock lows, with one low located where previous
investigations had indicated shallower bedrock. The ability to acquire seismic data in an active
industrial area where utility lines and sewer systems preclude using electrical or electromagnetic
methods helps ANAD to intelligently guide drilling, regulatory, and remediation strategies.
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Investigation Of An Earth Fissure Using Electrical And Seismic Methods
Authors Bjorn E. Sundquist and Barbara A. LukeEarth fissures present a hazard in the alluvial desert soils of the Las Vegas valley
(Nevada), particularly when their presence is masked by cemented surficial materials.
The authors performed electromagnetic conductivity, electrical resistivity, and surfacebased
seismic measurements over a known earth fissure lacking surface expression.
Preliminary reconnaissance using electromagnetic conductivity measurements indicated
the presence of a buried pipeline traversing the site. The presence of the fissure was
clearly indicated by the electrical resistivity measurements, which were performed in a
28-electrode dipole-dipole array, although the geometry of the fissure was not resolved.
Results were compared with a synthetic dataset. An experimental frequency-domain
surface-based seismic cavity detection investigation was also conducted over the fissure.
The method was moderately successful for this application.
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The Freshwater Resource Of Andros Island, Bahamas, Determined By Resistivity
Authors Angela L. Adams, Paul J. Wolfe, Cindy K. Carney and Mark R. BoardmanThe freshwater lens on Andros Island, Bahamas, is an important source of potable water for local
use and for export to the city of Nassau on New Providence Island. The production of large quantities of
water for export raises concern for the long-term sustainability of the resource. Resistivity soundings
provide a relatively easy way to evaluate and monitor the thickness of the freshwater lens. This
technique works well because there is a large contrast in resistivity between the freshwater and saltwater
saturated carbonate rocks that make up the island. Over the last 8 years we, along with students from
our universities, have conducted a number of resistivity surveys across the northern part of the island. A
number of problems, which complicate the acquisition of resistivity soundings on the island, will be
presented. Working around these problems we have developed a profile of the freshwater lens across
the island.
Our results show:
a single lens spans the island from Red Bays on the west to Conch Sound on the east,
the lens averages about 15 m thick over much of the island,
the lens thins to about 8 m under the well field areas, and
the lens thins to about 10 m in the vicinity of Charlies Blue Hole.
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Nuclear Magnetic Resonance Imaging Of Water Content In The Subsurface
More LessThe technique of nuclear magnetic resonance (NMR) imaging of subsurface water
content is described and critically evaluated from both experimental and theoretical
points of view. We describe data from soundings conducted in different parts of Colorado
and New Mexico, and describe constraints on the technique imposed both by
the present state of the instrument technology and by field conditions, such as cultural
noise, the subsurface geolectric section, pore saturation and pore size distribution, and
magnetic impurities in the soil. We describe also a newly derived general formula for
the NMR response voltage, valid for arbitrary transmitter and receiver loop geometry
and arbitrary conductivity structure of the medium in which the nuclear spins reside.
It is shown that in cases where the conductivity is large enough such that the electromagnetic
skin depth at the Larmor frequency is of the same order or smaller than
the measurement depth, there are diffusive retardation time effects that significantly
alter the standard NMR response formula used in the literature. Large quantitative
differences are found between conducting and insulating cases in physically relevant
situations.
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A Regional Tem Survey To Map Saline Water In The Cambrian-Ordovician Sandstone Aquifer Of Southeastern Wisconsin
Authors John Jansen, Robert W. Taylor and Ted PowellThe Cambrian-Ordovician aquifer is the major source of ground water for municipal and
industrial use in southeastern Wisconsin. Decades of over pumpage have created a regional cone of
depression centered in the western suburbs of Milwaukee. Total Dissolved solids levels in several
sandstone aquifer wells have more than doubled over the last ten to fifteen years. A time-domain
electromagnetic induction (TEM) survey was conducted to determine the cause of the rising TDS levels.
A total of 69 soundings were performed using a Geonics EM-57. The modeled soundings were
used to construct three geo-electrical cross sections across the survey area. The TEM survey indicated
that the rising salinity levels in the wells is related to vertical migration of saline water from the lower
aquifer in response to decades of over pumping. The findings are being used to evaluate future well
sites and rehabilitation options to reduce salinity levels in existing wells.
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The Effect Of A Conductive Surface Layer On Tem Central-Loop Soundings In Groundwater Exploration
More LessOne of the most popular geophysical methods of groundwater exploration is electromagnetic
sounding. In particular, central-loop configuration transient electromagnetic (TEM) soundings have
been shown to be quite effective in mapping horizontal strata of hydrogeological significance.
However, conductive surficial layers can interfere with what seems to be a straightforward application
of this method. Three effects of surficial conductive material can be addressed. First, the two-valued
apparent resistivity function becomes significant when surficial conductive layers are present,
necessitating a thorough understanding of this phenomena. Second, a breakdown in the definition of
apparent resistivity can occur in which there is no equivalent halfspace resistivity which would produce
the observed voltage in the measurement coil. Third, when collecting soundings near a lateral change in
surficial conductivity, migration of induced currents toward the conductor results in a sounding curve in
which transient voltages can drop very steeply, and can even change sign. This effect has been
explained in previous work in two ways. First, there can be a paramagnetic effect within soils near the
transmitter wire (mostly for the coincident loop configuration, but also for roving receiver locations near
the transmitter wire when using a large transmitting loop), and second, this can be due to an induced
polarization effect. A third explanation is developed here in which multiple current density rings are
induced in the subsurface. One ring corresponds to induction in material directly below the transmitter
loop, and a second ring can be developed non-symmetric with the transmitter loop, resulting from a
lateral surficial conductive layer.
These effects in TEM central-loop soundings are illustrated by analyzing data obtained at two
tidal flat environments, near Union Beach, and on the oceanside of Cape May Peninsula, New Jersey,
and from the surface of a lava flow resulting from the August 1999 eruption of Cerro Negro volcano,
Nicaragua.
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Electromagnetic Mapping (Em Map) Of Perched Water To Improve Remedial Activities At Hill Air Force Base
Authors T. Phillips and M. CoxThis report contains a description of how a proprietary geophysical technique was used to assist
groundwater investigations, in and around Operable Unit 1, OU1, located on the eastern side of Hill Air
Force Base, (HAFB), Utah. The primary objective was to define the sources of a groundwater plume
contaminated with Light Non Aqueous Phase Liquids, LNAPLs, which originated within OU1. An
additional objective was to locate subsurface channels for the plume that extended into off-base areas.
Although investigations have defined 7 individual groundwater bearing zones, most of these
investigations focused on the near-surface water, i.e., about 20 ft below the surface, groundwater
flowing horizontally on top of the clays of the Alpine Formation and within the sands and gravels of the
Provo Formation.
Results of the geophysical surveys defined water channels feeding seeps along the east and north
sides of two landfills within OU1. An additional survey defined sources for groundwater recharge.
Channels contributing to the off-Base groundwater were mapped in a separate survey. The technique
also proved useful in determining groundwater sources for off-Base landslides. A final survey was able
to confirm that water in the lower water-bearing zones of the Alpine Formation was not connected to the
near-surface groundwater plume.
The paper defines how this proprietary geophysical technique, based on U. S. Patent #5,825,188,
is a cost-effective means of defining groundwater flows. When coupled with the other investigative
tools used at OU1, including monitoring well/piezometer installations, Cone Penetrometer Tests (CPT),
and soil borings, the technique improves the overall understanding of groundwater flows at the site and
provides for a higher degree of confidence in the siting of other investigative tools. Examples are
provided showing where a groundwater channel feeding a seep, missed in a large drilling/CPT program,
was identified. In addition, the technique also provided essential data for the design of a groundwater
interception trench to connect contaminated groundwater and LNAPLs.
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Geophysical Surveys For Groundwater In The Gaza Strip
Authors W. Ed Wightman, James Hild and Steffen HodgesThe Gaza Strip (Gaza) is an area 40 km long and 8 km wide along the eastern shore of
the Mediterranean Sea. The primary sources of groundwater in Gaza are shallow aquifers
within the Kurkar formation, a transgressive-regressive sedimentary section consisting
primarily of interbedded sands and clays. The Kurkar rests on a marine clay called the Saqiye,
which forms the base of the aquifer. The quantity of fresh water that can be withdrawn
depends on a number of factors, such as recharge, geology, and hydraulic properties.
Moreover, since the aquifer is in contact with seawater, interfaces between fresh and saline
water are in dynamic equilibrium, and the distribution of the interface is a critical parameter
defining the resource. The water table in Gaza is at an elevation of about sea level.
Groundwater characterization studies of the Kurkar aquifer have been an ongoing
project in Gaza, funded by USAID. Recently, geophysical surveys were used to better define
the shallow stratigraphy and groundwater salinity distribution within the Kurkar aquifer.
Various non-intrusive geophysical methods were tested for this purpose, however the best
results were derived from an integrated interpretation of time domain electromagnetic
soundings (TDEM) and seismic reflection data. The scope of work consisted of acquiring,
processing, and interpreting 24.7 km of seismic reflection data, and 84 TDEM soundings. A
small amount of Induced Polarization, resistivity and seismic refraction data was also acquired
but did not aid significantly in the interpretation.
The salinity of ground water within a geologic unit can often be inferred from
resistivity, since it is strongly influenced by salt concentration that lowers groundwater
resistivity. However, clay horizons also often have low resistivities, and can be misinterpreted
as saline water. On the other hand, seismic reflection data images the lithologic boundaries
between sand and clay layers, and is insensitive to groundwater salinity. Thus if a lithologic
boundary is coincident with a resistivity boundary it is more likely to correspond to a clay layer.
On the other hand a low resistivity region which is not defined by a seismic reflector may
simply reflect a change in the resistivity of the pore water. Therefore, by combining the two
methods it is possible to better interpret the cause of low resistivity values. Several wells close
to seismic lines were available with geologic logs allowing the correlation of the geophysical
data with the geologic logs.
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Improving Understanding Of Peatland Hydrogeology Using Electrical Geophysics
Authors Lee D. Slater, Andrew Reeve and Danney GlaserA geophysical survey was completed in Caribou Bog, a large peatland in Maine, to evaluate
peatland stratigraphy and hydrology. Geophysical measurements were integrated with direct
measurements of peat stratigraphy from probing and with measurements of fluid chemistry. Consistent
with previous field studies, GPR was an excellent method for delineating peatland stratigraphy.
Prominent reflectors from the peat-lake sediment and lake sediment-mineral soil contacts were precisely
recorded up to 8 m deep. However, GPR provided no information below the mineral soil contact. 2D
resistivity and induced polarization (IP) imaging was used to further investigate the stratigraphy of this
peat basin. We observe that the peat is chargeable and that IP imaging is an alternative method for
defining peat thickness. This chargeability is attributed to the high surface charge density on partially
decomposed organic matter. The conductivity imaging resolved glaciomarine sediment thickness and its
variability across the basin. Terrain conductivity measured with a Geonics EM31 correlated with
glaciomarine sediment thickness and was effective in characterizing variability in layer thickness over
approximately 18 km2. The electrical imaging indicates that variations in glaciomarine sediment
thickness may exert a key control on the hydrogeology and vegetation distribution within this peatland.
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Applications Of Shallow Geophysics In A Regional Geological And Hydrogeological Investigation, Oak Ridges Moraine, Southern Ontario
Authors S.E. Pullan, A. Pugin, J.A. Hunter, S.D. Robinson, M.A. Annecchione and G.E. LeblancGeophysics has played a critical role in a multi-disciplinary research program in the Oak Ridges
Moraine area, southern Ontario, designed to improve understanding of the regional geological and
hydrogeological framework of a significant Quaternary aquifer complex. Geophysics was particularly
important because of the large study area, complex glacial stratigraphy, and thickness of overburden
sediments (up to 200 m). Borehole, surface and airborne geophysical surveys have provided information on
the subsurface at scales of investigation varying from sub-metre to kilometre. A suite of geophysical logs
were obtained in eleven, deep (90-190 m) stratigraphic boreholes. These data were particularly important
in identifying downhole stratigraphic relationships and effecting regional correlation. Ground probing radar
was used extensively in glaciofluvial sand and gravel deposits of the ORM to obtain depth to water table and
very-near-surface structural and sedimentological information. Electromagnetic techniques were tested, but
not used extensively during the project. Over 50 line-km of land-based, shallow seismic reflection profiles
provided a means of investigating the subsurface architecture and stratigraphic relationships of the complete
sequence of unconsolidated sediments. These surveys were instrumental in identifying the regionally
extensive and eroded nature of the Newmarket Till beneath the ORM deposits. Data from a regional gravity
survey consisting of over 5500 stations were inverted to obtain an interpretation of bedrock topography.
Though as yet unproven, high-resolution airborne magnetic data have identified anomalies which may be
related to channel features within 200 m of the surface. Overall, the combined geophysical data have
provided high-quality control for a large set of archival data, and have allowed an assessment of the lateral
continuity of major hydrostratigraphic units. This paper outlines the objectives and applications of these
geophysical surveys in the Oak Ridges Moraine study, and assesses their potential applications in other
regional hydrogeological investigations.
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High-Resolution Seismic Survey To Evaluate Groundwater Potential At A Proposed Electric Power Generation Facility
Authors William Black, Richard Miller and Richard PalmHigh resolution seismic reflection profiling was effective in delineating geologic units within the
upper 200 ft at a proposed power plant site in southern Minnesota. Glacial activity is responsible for the
topography of the bedrock surface evident on seismic reflection data across this site. Gravel and sands
deposited during glacial retreat into bedrock lows can provide a significant source of water for a variety
of residential, commercial, and municipal applications. Selection of power generation plant sites is
routinely based on availability of fuel, proximity to existing power grid, and a ready source of water. In
this case the first two criteria were met; the purpose of the seismic survey was to evaluate and direct a
drilling program that would establish the third. Dominant frequencies of the nominal 24-fold data
acquired along two lines are, on the average, in excess of 150 Hz, which provides practical vertical bed
resolutions (~½ wavelength) on the order of 15 ft. Bedding geometries interpreted from CMP stacked
sections were consistent with several boreholes located around this 80-acre site. Several apparent structural
features interpreted at bedrock depth on the stacked sections were mapped as potential water
sources. The bedrock low interpreted from drill data was observed on the stacked seismic section but
was confined to a much smaller area than mapped using the few scattered drill holes. A complex bedding
geometry associated with the advance and retreat of glaciers is evident on the seismic data above
the bedrock surface. Drilling confirmed the seismic interpretation and provided additional information
about the nature of the material within the bedrock lows. This proposed site was abandoned due to insufficient
quantities of gravel and coarse sands within the glacially sculpted bedrock lows.
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Geophysical Mapping Of Groundwater Potential In A Rural Water Supply Project: Malawi, Africa
Authors Richard L. Kellett, Jim R. Anscombe, Paul D. Bauman, Philip Hankin and Leon EngelbrechtA total of 250 boreholes were sited in the East Mangochi District of Malawi, using a unique
combination of borehole geophysics, terrain conductivity, frequency domain electromagnetics, and
electrial resistivity tomography. The project represents one of the most intense applications of
geophysical surveys to rural water supply in deeply weathered crystalline basement terrains. A total of
388 line kilometers of EM, 22 line kilometers of ERT, and 23 borehole logs were collected in a threemonth
program in 1999. These data have been processed and interpreted in conjunction with previous
hydrogeological reports and satellite and airphoto interpretations, to produce over 480 targets for lowyield
village hand pumps. Such a detailed geophysical survey was considered necessary in the region
because of the highly irregular distribution of aquifers and the low success rate of past drilling programs.
The EM surveys proved to be the most efficient in terms of rapid surveying, but the ERT sections were
essential for resolving ambiguities in the EM response of lateral and vertical resistivity changes. Water
saturated deep weathering zones, and vertical fracture sets were the most commonly mapped targets
having strong responses in both the EM34 and the Apex MaxMin systems. Resistivity sections across
these features accurately mapped the depth to unweathered bedrock, the presence of a saprock layer, and
the thickness of the shallow laterite. Data management and interpretation was aided by the use of a
Geographical Information System (GIS) that allowed continual monitoring of the survey, and rapid
access to the results. Drilling of the boreholes commenced in September 2000 using the Global
Positional System (GPS) locations, village maps, and physical markers on the ground to locate the
targets. To date, of the 35 wells drilled, 27 have been completed as successful water wells.
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Natural Aggregate -- Geophysical Opportunities
Authors William H. Langer and Karl J. EllefsenDuring the early part of the 20th Century, the increased demand for surfaced roads prompted a
number of reports in the geologic literature on using aggregate for highway construction. Since that
time many other excellent reports concerning research, development, testing, and evaluation of natural
aggregate - crushed stone, sand, and gravel - have been written. However, most professional geologists
and geophysicists, even those who work in the mineral-resource field, commonly ignore natural
aggregate in favor of other seemingly more glamorous aspects of geology.
Today, at the beginning of the 21st Century, we recognize the indisputable need for a continuing
and uninterrupted supply of high quality aggregate. The United States annually produces over 2.5
billion tons of aggregate, which represents approximately 9 tons per person per year. During the next
decade, the restoration and rehabilitation of an aging infrastructure will require enormous amounts of
aggregate, much of which will need to meet strict specifications for new high-performance construction
materials. In addition, stringent environmental laws, land-use conflicts, zoning restrictions, and the "Not
in my back yard" syndrome make it increasingly difficult, time-consuming, and costly to obtain permits
for extraction of aggregate.
Geophysicists have the opportunity to provide critical information concerning the identification,
characterization, and extraction of suitable aggregate, and concerning protection of the environment
from adverse impacts of aggregate extraction. Geophysical surveys of aggregate deposits also provide
opportunities for conducting traditional research on geophysical methods. Research is needed on
techniques to efficiently acquire and process ground-based geophysical data, airborne geophysical data,
and remotely sensed data. Such data are needed to detect and delineate potential sources of crushed
stone and deposits of sand and gravel. Better techniques are also needed to determine deposit thickness,
characterize variations within a deposit, characterize physical properties of aggregate resources,
characterize mine sites, and characterize areas of potential environmental impacts.
It is time to shift paradigms, to implement new ideas and methods, to get involved in aggregate
research. In doing so, the inquisitive geophysicist will soon recognize numerous challenging
opportunities that make natural aggregate a subject as fascinating and rewarding as any other aspect of
geophysics.
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Mapping Of Bedrock Aquifers In The Denver Basin Using Borehole Geophysical Logs
Authors L.R. Arnold and S.G. RobsonGround water is a natural resource that is present in bedrock aquifers that underlie much of the
Front Range Urban Corridor in Colorado. In 1996, mapping of bedrock-aquifer outcrop and subcrop
areas along the western margin of the Denver Basin was undertaken by the U.S. Geological Survey in
cooperation with the Colorado Department of Natural Resources, Division of Water Resources, and
the Colorado Water Conservation Board. Detailed mapping of outcrop and subcrop areas of the Denver
Basin bedrock aquifers was done to better define areas where the aquifers receive recharge
directly from precipitation at the ground surface and where they are in direct hydraulic connection
with alluvial aquifers. Geophysical logs from oil, gas, and water wells and lithologic logs from
exploration borings were used to map the subsurface structure and thickness of the bedrock aquifers.
Structural contour maps of the bedrock aquifers then were used in combination with topographic
maps of the land surface, information from field observations, and published geologic maps to determine
where bedrock-aquifer outcrop and subcrop areas occur. The results of the study indicate that 1)
the bedrock aquifers along the western margin of the Denver Basin generally dip gently eastward into
the basin and have steepening dips in the vicinity of the mountains of the Front Range and 2) the
width of bedrock-aquifer recharge areas varies greatly as a function of dip. The presence of deltaplain
paleochannels was inferred from a thickness map of one of the bedrock aquifers. The study
demonstrates that geophysical logs can be a useful tool in mapping the subsurface structure of bedrock
aquifers, especially in areas where it is difficult to distinguish between formations on the basis of
lithology and in areas where bedrock is covered by a thick mantle of unconsolidated sediments.
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