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17th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 22 Feb 2004 - 26 Feb 2004
- Location: Colorado Springs, Colorado, USA
- Published: 22 February 2004
81 - 100 of 165 results
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Rpm – Residual Potential Mapping; A New Approach To Mise-A-Lamasse
Authors Christopher A. Baldyga and James B. FinkBetter methods are needed for the detection and mapping of low-contrast hydrogeophysical
targets. Several publications within the last two decades show merit in the application of the mise-a-lamasse
method to hydrologic problems such as fracture flow, plume detection, and plume migration.
This presentation suggests a modification to the conventional approach that will enhance low contrast
targets.
The mise-a-la-masse method is a simple application of electrical methods where a buried
conductive feature is electrically energized and the resulting distorted electric field on the ground
surface is mapped. Mise-a-la-masse has been in use for decades in the mining industry on massive
sulfide orebodies, but, the requirement that the orebody be both massive and electrically contiguous has
limited the method’s application.
The normal interpretation of mise-a-la-masse data depend on the ability to observe the desired Efield
distortions caused by the energized target. In the case of low physical property contrasts,
distortions can be very difficult to see, if at all. The primary field observed in the proximity of the
energizing electrode dominates the response. Unless the target is particularly conductive and-or rather
shallow, it will not produce an E-field distortion distinct from the primary field. The observed E-field
consists of the superposition of several electrical responses.
The observed E-field at the ground surface is a combination of all of the effects created by the
energized target and various heterogeneities, conductive or resistive, that influence the distribution of
the current flux. These various effects may be due to geometry, such as layering; lateral paths, such as
fractures; and localized high-contrast features, such as culture.
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Complex Inversion Of Surface-Nmr Signals - Extending The Limits Of Model Resolution
Authors Martina Braun, Marian Hertrich and Ugur YaramanciThe technique of Surface Nuclear Magnetic Resonance (SNMR, also known as MRS - Magnetic
Resonance Sounding) is used for direct groundwater determination and aquifer characterization. Among
other influences, the electrical conductivity of the subsurface leads to a complex-valued signal. However,
the standard interpretation scheme uses only the amplitude of the signal for determining the water content.
But real and imaginary parts of the signal are sensitive to different depth volumes. Generally, the imaginary
part is more sensitive to deep structures than the real part of the signal, i.e. in conductive media, signals
arising from deep layers have a significantly greater imaginary part than an equivalent signal from shallow
depths. For real data, physical effects additionally to the electromagnetic phase delay have to be considered
and adequately quantified to use the phase information for an enhanced data interpretation. This study
assesses the complex inversion using real and imaginary parts of the signal. Analyzes of synthetic and real
data with sufficient data quality show that the complex inversion is more reliable in terms of determining
deep structures, equivalence errors are reduced, and the depth resolution is increased.
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Inversions Of Surface-Nmr Signals Using Complex Kernels1
Authors Martina Braun and Ugur YaramanciThe SNMR (Surface Nuclear Magnetic Resonance, also called MRS - Magnetic Resonance Sounding)
method is used for direct groundwater exploration and improved aquifer characterization with measurements
on the surface. Since the electrical conductivity of the subsurface alters the SNMR amplitude
and especially the phase, the conductivity structure must be known before the inversion of the SNMR
amplitudes. This study demonstrates the effect on the water content determination if the electrical conductivity
is not considered. Disregarding the electrical conductivity results into significant wrong water
content distributions even at moderate resistivities of 100 m. The conductivity must be taken into account
down to an appropriate depth, also structures below the assumed penetration depth of 100 m can
effect the water content determination. Non considering conductive structures above or in the aquifer results
into an underestimation and downward shift of the inverted water bearing layer; conductive media
below the aquifer lead to an overestimation of the water content. For real data, both amplitude and phase
values should agree with modeled data. Analyzing the SNMR phase values of real data provides additional
information for estimating the reliability of the inversion result.
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Assessing Groundwater Perching Horizons Using Synthetic, Ground, And Airborne Tdem Data At The Pantex Plant, Texas
Authors Jeffrey G. Paine, S. Todd Harris and James M. PhelanAt the U.S. Department of Energy's Pantex Plant near Amarillo, Texas, recharging groundwater
encounters a perching fine-grained zone (FGZ) above the main Ogallala (High Plains) aquifer, a critical
agricultural, municipal, and industrial water supply. We conducted modeling and ground-based timedomain
electromagnetic induction (TDEM) measurements to examine whether TDEM might help assess
the integrity of the FGZ and its ability to retard the flow of groundwater to the main aquifer. Conductivity
models based on well logs and representative TDEM soundings demonstrated that changes in thickness
of the FGZ and perched aquifer are detectable if changes in other stratigraphic horizons are minimal.
Based on these results, we conducted an airborne geophysical survey using Fugro's GEOTEM system.
Conductivity-depth transforms (CDTs) from airborne measurements matched the nearest ground-based
profiles reasonably well. Pseudo-depth slices constructed from CDTs depicted apparent conductivity at
critical depths, enabling refinement of borehole-based stratigraphic and hydrologic models. Elevated apparent
conductivities were observed where interpreted FGZ integrity was good and the saturated zone is relatively
thick; low apparent conductivities are interpreted to indicate areas where the FGZ and associated saturated
zone are thin or absent. Survey results are being used to support groundwater investigations, development
of fate and transport models, and upcoming corrective action decisions.
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Developing Recharge Reduction Strategies In The Tintinara Area Of South Australia Using Resolve Frequency Domain Helicopter Electromagnetic Data
Authors Tim J. Munday, Ross C. Brodie, Andy A. Green, Steve Barnett, Fred W. Leaney and K.P. TanIn the Tintinara area, located south-east South Australia, airborne geophysics was recognized as
having potential to provide valuable biophysical data relevant to the management of irrigation
development and groundwater recharge reduction in the area. The groundwater of the area sustains
irrigation and other dryland agriculture. However the lifetime of this resource is limited by the leaching
of salt that has accumulated in the soil prior to land clearing and agricultural development. For some
areas, the groundwater may be saline and unusable for irrigation within ten to twenty years. The
presence or absence of a near surface clay unit can have an important influence on the rate and timing of
this deterioration by slowing recharge. Forward modelling suggested that a frequency domain helicopter
electromagnetic (HEM) system could map spatial variability associated with this unit. A survey was
conducted using the RESOLVE® HEM system and demonstrated that, through the use of a constrained
inversion approach, this unit could be mapped. This was confirmed with shallow drilling. We have
generated a product, namely clay thickness, which is now being used as an input into a hydrogeological
model to help predict recharge rates and influence management decisions in the area.
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Constrained Inversion Of Helicopter Aem Data For Managing Irrigation Salinity
Authors Andy Green, Ross Brodie and Tim MundayThe RESOLVE frequency domain HEM system has been used to map the distribution of near-surface
clay-rich sediments in and around the Riverland irrigation districts of South Australia.
Approximately 12,000 line-km was surveyed at line spacings of 150 or 300 m. The data were
recalibrated with measurements from down-hole induction logs and then inverted using a 1-D layeredearth
model. In order to improve the sensitivity to the unknown aspects of the section, the inversion was
constrained with as much local geological and hydrologeological information as possible. These
constraints included information about the depth of the water table, the conductivity of the groundwater,
the variability of the conductivity and thickness of three sedimentary units, and the geomorphic history
of the area.
The resulting detailed map of the distribution of the Blanchetown Clay is being used to model the
recharge behavior of the area, which in turn can be used to help predict the future course of salinity
inflows to the River Murray. If more areas are to be released for irrigation, the map could be used to
select areas of thicker clay that are preferred locations for such developments.
The results of the inversion also allow us to reconstruct the strandline-dominated paleo-topography left
when the sea retreated from the Murray Basin in the Early Pliocene. The survey also revealed a hitherto
unsuspected, deeper variability in conductivity following the Pliocene strand line pattern. The cause of
this pattern is not clear. It could be due to variation in the porosity of Loxton Sands or to strandlinecorrelated
variability in the elevation of the contact between the Bookpurnong Beds and the Lower
Loxton sands.
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Bundaberg Groundwater Investigation, Australia – A Case For The Benefits Of Extensive Use Of Geophysics In Groundwater Investigations
Authors Geoff Pettifer and Warwick WoodIn 1998/99 the Department of Natural Resources (DNR) in Bundaberg, Queensland,
Australia contracted Geo-Eng Australia (now GHD Pty Ltd), to undertake a major joint study of the
coastal groundwater system of the Bundaberg Irrigation Area (BIA) which was experiencing
problems with the unregulated use of much of the aquifer and seawater intrusion problems. The
Bundaberg Groundwater Project is arguably the largest integrated geophysical, drilling and
hydrogeological project for water resources assessment undertaken in Australia.
The Project was a pre-cursor for a proposed new BIA groundwater model. One primary
project objective was to acquire sufficient data on the extent and properties of the groundwater
system to redefine the conceptual hydrogeological model. Another primary objective was to
systematically apply integrated geophysical, geological and hydrogeological techniques and gather
permeability data, directly and empirically, for identification of permeability trends.
The project provides a case study, showing the value of a strong commitment to the largescale
use of routine and innovative geophysics throughout a major groundwater investigation.
Firstly, the project budget allowed use of ground resistivity (of 702 kms of traversing and
273 soundings), seismic reflection soundings and multi-parameter geophysical logging (270
new/existing holes) on a scale generally not contemplated in groundwater studies. The extensive
geophysics guided a major drilling program of 130 new holes including 106 new piezometers
optimised in position and depth.
Secondly, it showed the value for groundwater management decision-making of an integrated
analysis of disparate datasets (airborne and ground geophysics, hydrogeological, topographic,
hydrochemistry and geophysical logs).
Thirdly the application of sequence stratigraphic analysis techniques, to geophysical log data
defined a new conceptual hydrogeological model and understanding of the hydrogeological context
and information value of each existing and new piezometer.
Finally, the geophysical logs assisted by the surface geophysics, proved useful in providing
an assessment of permeability trends for groundwater model building.
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Geophysical Methods Applied To Saltwater Intrusion In Antigua
Authors Stewart K. Sandberg and Hastin BarnesThe island of Antigua has been suffering from an acute shortage of potable water due largely to
drought conditions on the island. Because of the persistent drought conditions one of the island's
primary sources of potable water, the Potworks Dam, is dry. In addition to the Potworks Dam, the
nearby smaller Collins Dam is also dry. In addition to these dry surface impoundments, the problem is
further complicated by the fact that the reduced recharge has resulted in a lowering of the watertable in
the nearby Collins abstraction region. As a consequence, salinity levels in abstraction wells at Collins
have increased. In extreme cases, the watertable has declined below the depth of the abstraction pump.
A geophysical survey was conceived to generate information related to the hydrogeologic
framework of the area, while taking advantage of the simplified logistics resulting from the dry
conditions of Potworks reservoir. During the period from 14 April to 21 April, 2003, geophysical data
were acquired in the dry bed of the reservoir, and at two locations east toward the coastline.
Geophysical methods included EM-31, Schlumberger array resistivity soundings, and TEM central-loop
soundings.
Geophysical results show lateral variation in subsurface resistivity related to both lithologic
variation and groundwater salinity. Shallow resistivity variation, mapped with the EM-31 system, is
indicative of a channel system within the bed of the Potworks reservoir. Variation in resistivity mapped
using the TEM and resistivity soundings likely indicates structure of the Karst aquifer, which has
variable resistivity due to porosity/salinity variation.
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Seismic Anisotropy In The Edwards Formation: Government Canyon State Natural Area
More LessHigh resolution seismic data are acquired from the Edwards limestone formation in an area of
outcrop within the Government Canyon State Natural Area, to infer the local orientation of vertical
fracturing. The experiment consists of a single 3D seismic array of 16 3C recording stations. Seismic
sources are deployed around and within the array at short spacings resulting in 16 3C source gathers
(by reciprocity) with thousands of traces. A velocity-variation-with-azimuth trend is discernible in
the data attributable to azimuthal anisotropy due to vertical fracturing. A slow direction along
120 degrees from North is found, with a fast direction along 20 degrees from North. If vertical
fracturing and seismic anisotropy are correlated at this site, a fracture orientation along 20 degrees
from North is determined.
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Practical Application Of Geophysical Mapping For Wellfield Exploration - Two Indiana Case Studies
Authors Gregory B. Byer and John VanderlaanMany communities in the State of Indiana rely on the availability of groundwater to sustain and
support population growth and new development. Although Indiana generally experiences plentiful
precipitation throughout the year, many communities were founded in localities where potential
groundwater production is limited by geologic factors. The northern two thirds of Indiana is covered by
glacial drift, which is often dominated by glacial till, a material that is generally unsuitable for
community water supply purposes due to both poor water quality and potential yield. At locations that
are both sited on glacial till and are not located near a significant source of surface water, the only other
alternative source of groundwater is fractured bedrock. And in fact many locations are able to utilize
fractured carbonate or sandstone aquifers.
However, some communities have no significant source of surface water and are underlain by
drift dominated by glacial till, which rests upon non-aquifer bedrock such as shale or siltstone. Few
good options remain in such a case, and some communities must import water from another location.
However, some of these communities do have isolated, coarse-grained water laid glacial outwash
deposits available to them, and due to the complexities of the glacial depositional environment, the
occurrences of these sand and gravel deposits are difficult to ascertain, and test drilling to find them
using a "hunt and peck" method can become cost prohibitive. In such cases geophysics can fill an
information gap. Two case histories in Indiana are presented here where terrain conductivity and
2-dimensional resistivity have been used to tackle this difficult problem in cooperation with both
municipal engineers and drilling contractors. Both examples illustrate the effectiveness and resulting
broad understanding and acceptance of these methods, and demonstrate the potential for the use of
geophysics to become the norm in wellfield exploration in these types of situations.
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Bedrock Water Supply Exploration Using Very Low Frequency Geophysics
Authors Christopher L. Covel and Darryn T. Kaymen-CovelVery Low Frequency (VLF) geophysics has consistently proven to be a successful method for
groundwater supply exploration in locating bedrock wells. The use of geophysics in solving difficult
and evolving water supply demands allows for the successful use of this technology to ever broadening
markets as restrictions and new requirements on water supply withdrawals increase. VLF geophysics is
a proven scientific technology used to locate the highest yielding bedrock water wells at a site by
identifying specific water bearing fractures before the actual drilling is performed. In this case study
VLF geophysics was applied at a site in central New Hampshire to locate a high yielding water supply
well for a golf course. For a proper VLF investigation to be performed a VLF transmitter must be
located approximately on strike or trend to the particular fractures or faults that are to be studied.
Knowing the bedrock fracture pattern is important, because the geophysical line must be laid out more
or less perpendicular to the VLF transmitter selected. This is critical for the collection of meaningful
data. The central New Hampshire site bedrock consisted of crystalline granite and metamorphic
bedrock which locally is known to produce low yielding wells. The investigation consisted of existing
data review, fracture trace/lineament analysis, bedrock mapping, line layout, collection of VLF data and
data interpretation. This bedrock well yielded 80+ gallons per minute (gpm). Alternatively, for the site
to withdraw 80 gpm from a local surface water source extensive permitting, construction and an
unpredictable amount of time would have been required. This example documents how VLF geophysics
is an accurate and cost effective solution to ever increasing water supply problems faced on the earth.
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An Overview Of The Inyo County’S 2003 Death Valley California Area Geophysical Program
Authors John R. Jansen, Michael J. King, Joy Loughry, Ted Powell and Doug LaymonThe relationship between carbonate waters from the Texas, Travertine, and Nevares springs in
Death Valley (within the Southern Funeral Mountain Range) and the groundwater flowing under Yucca
Mountain has yet to be conclusively demonstrated. Currently there is only one accessible monitoring
well that has penetrated the Lower Carbonate Aquifer near Yucca Mountain. There is only one
accessible well that penetrates the Lower Carbonate Aquifer down-gradient of Yucca Mountain.
The U.S. Department of Energy has awarded Inyo County grant funds for a geophysical and
drilling program to evaluate the connection between the Lower Carbonate Aquifer (LCA) in the
Amargosa Valley and the Southern Funeral Mountain Range. The program consists of a series of
geophysical surveys to characterize the geological structure of the LCA in the Southern Funeral
Mountain range, and the drilling of five monitoring wells into the LCA.
In FY2002 and FY2003, a total of 59 miles of gravity data were collected in the Armagosa
Valley near Bat Mountain in the Southern Funeral Mountains. The interpreted LCA surface shows a
complex topography with steep dips and several probable faults. The data was used to select drilling
locations for four monitoring wells to be drilled at depths of 1,000 to 4,000 feet.
Approximately 9 miles of continuous magnetic data and 8 TEM soundings were collected in the
Scotty’s Junction area of Sarcobatus Flats to map the Sarcobatus Fault as part of a water resources
investigation being conducted by the USGS on behalf of the National Park Service. Previous mapping
had indicated several possible locations of the fault. The magnetic data indicated the presence of a fault
system of at least three faults. The TEM soundings indicated that the faults were steeply dipping normal
faults.
This work is a continuation of surveys we conducted in previous years. The previous surveys
included TEM soundings at the Travertine, Texas, Nevares, and Grapevine Springs to determine the
hydrogeologic controls influencing the location of these springs. Future geophysical efforts will
concentrate on providing additional subsurface control to help quantify the groundwater discharge to the
saltpan of Death Valley.
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Groundwater Research Within The Estancia Basin, New Mexico
More LessGroundwater shortage is a serious issue within the Western United States. Controversies arise
when long-standing farmers and ranchers dispute with new residents moving in from suburban areas
concerning water rights. Vern Wood, resident and former member of the Edgewood Soil and Water
Conservation District brought his community’s concern to the Colorado School of Mines. Wood lives in
the Estancia Basin of New Mexico and identified three critical issues facing his community: the
characterization of the Estancia water aquifers, the quantification of the aquifers’ water volume, and the
recharge areas for the aquifers.
Mr. Wood prompted the 2003 Colorado School of Mines Field Camp to take place in Edgewood,
New Mexico in an effort to begin collecting data on the Estancia Basin. The intricacies of the
groundwater situation lent themselves well to a collaboration between ranchers, students, faculty, and
industry. Companies such as: Veritas, Sercel, Geonics, Newmont, and Zonge donated geophysical
equipment, giving Mines students and faculty the opportunity to work alongside local USGS experts,
professors, water groups, and residents using a wide array of geophysical methods. The geophysical
techniques employed to study the fracture system of the aquifers, included: surveying, deep seismic
reflection and refraction, well-logging, numerous DC resistivity methods, gravity, electromagnetic
induction (both time and frequency domain), VLFR, and shallow seismic reflection. The results of the
study included large amounts of data, and the opportunity to go back for further investigation.
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A Subway Project In Lausanne, Switzerland, As An Urban Microgravimetry Test Site: Acquisition, Correction Of Buildings Influence And Modeling
Authors Pier Vittorio Radogna, Raymond Olivier and Philippe LogeanA new subway will be constructed within the next decade in Lausanne. The geology involved
constitutes alpine molassic bedrock and an overlaying quaternary glacial fill. The choice of the subway
corridor (6 km) had to consider the exact thickness of these geological units. The availability of
mechanical logging data offered the opportunity for testing a microgravimetry survey. The distance from
nearby constructions determined whether corrections had to be made for these volumes or could be
omitted. We calculated, with an in house program, the effect of different buildings to choose
architectural styles with similar gravimetric responses. We are using then a geographic information
system (GIS) to group these buildings by typology based on cadastral records and correct all the
buildings with simplified architectural styles. The voids of the basements were included in the digital
elevation model to take account of the complexity of the Lausanne morphology. All these corrections
could be calculated before the acquisition in order to chose the best gravity stations location. The
residual gravity map is adjusted to only some drillings, in order to regionalize this punctual data and to
compare the results of the bedrock modeling with the drillings not taken into account.
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Locating Pipelines With A Broadband Electromagnetic Induction Array System
Authors Ralf Birken, Thorkild Hansen, Ross Deming and Michael OristaglioAn electromagnetic induction system for locating buried pipelines is being developed in
collaboration with utility companies under the United States Department of Transportation, Research
and Special Programs Administration’s Pipeline Safety Research and Development Program1. The
components of the system are (1) an array of broadband vector magnetic field (induction) sensors,
(2) transmitters at various (discrete) frequencies, (3) a laser positioning system, and (4) inversion
algorithms.
The system is designed to locate buried pipelines by creating a digital map of the vector
magnetic field generated by currents flowing along the pipelines (and in the surrounding soil). The
currents can be set up either by injecting current directly onto individual pipelines (where accessible)
with galvanic or toroidal current sources or by inducing currents remotely with a surface source. When
injecting currents directly to track several pipelines, each “clamp-on” source can inject current at a
different frequency within the spectral range of the broadband sensors (about 1 kHz to 50 kHz). In
principle, this allows tracking of individual pipelines by decomposing the recorded magnetic-field time
series into its frequency components. An inversion algorithm models the magnetic field with a system of
current filaments flowing along a network of underground pipelines. Current flowing along each
pipeline is represented by one or more (straight) line filaments, which can intersect with other lines; the
amount of current along the filament is allowed to vary linearly between its endpoints (nodes) to
simulate current leaking into the soil. Conservation of current is enforced at junctions between line
segments representing a pipeline (or at junctions between pipelines). The algorithm iteratively
determines the three-dimensional (3D) location of each current filament and the current at its nodes by
minimizing a cost function.
The system uses a self-tracking laser theodolite to monitor the position of the sensor array at
regular intervals during operations. The array can move along arbitrary paths (for example, in traffic) to
cover the area to be surveyed; special algorithms merge the data from different paths to create the
complete digital map of the surface magnetic field. Even without further processing, the digital map
itself shows useful information about the locations and shapes of underground features.
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Seismic Imaging To Accurately Characterize Subsurface Ground Conditions
More LessUnforeseen, variable subsurface ground conditions present serious challenges to the heavy
construction and civil engineering industry to design, construct, and maintain infrastructure.
Detailed, accurate knowledge of ground conditions reduces project risk, improves construction
performance and safety, prolongs structure life, and prevents waste in over-design. All forms of
major construction projects are impacted; including tunneling and microtunneling (transportation and
utility), foundations and structures (buildings, piers, runways, retaining walls), bridges and highways
(abutments, roadcuts, slopes), and dams and impoundments. Presently, site characterization and
geotechnical engineering, supporting these activities, are limited by the inability to adequately
describe these subsurface ground conditions.
NSA Geotechnical Services has successfully applied seismic tomography and holography
ground imaging technologies on tunneling and heavy civil construction projects worldwide. Seismic
signal waveforms traveling through a complex medium consist of various arrivals from refractions,
reflections, scattering, and dispersion. Tomography and holography are proven inversion
technologies for estimating location and extent of material property variations causing changes in
signal waveforms.
The basic principle behind RockVision3D™ is that seismic energy travels through different
material types with different attenuation rates and velocities. That is, seismic waves travel more
quickly, with less attenuation, in strong, competent material. Seismic waves travel more slowly,
with more attenuation, in weaker materials (e.g., voids, broken or weathered rock, soil).
The basic principle of TRT™ is that a portion of the seismic energy traveling through the
earth is reflected when it encounters an interface between ground zones possessing different seismic
properties. Most geologic structures, anomalies, and changes in lithology provide detectable seismic
reflections if they are within a reasonable distance of the seismic source.
This paper will present various applications of these technologies, illustrating how seismic
imaging can provide accurate information regarding ground conditions. With this information,
engineers can complete projects safely, within time and budget constraints.
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Data Collection, Processing And Analysis Challenges—Gpr Bridge Deck Deterioration Assessment Of Two Unique Bridge Deck Systems
Authors Francisco A. Romero and Roger L. RobertsBridge deck deterioration mapping using the top rebar reflection amplitude as a measure of
deterioration is a useful tool for rapid, non-invasive bridge deck condition assessment. Currently, the
technique is applied using either a dual-polarization horn antenna setup or a ground-coupled antenna.
Isolating the top rebar reflection in a bridge deck requires considerable planning and knowledge of the
bridge deck structure.
This paper presents the data collection, processing and analysis issues associated with the
deterioration assessment of two unique bridge deck systems located in Des Moines, Iowa. Some
considerations weighed during the selection of this GPR methodology will be discussed, with
comparisons made to other data collection/processing alternatives. Slight modification to the typical
analysis methodology was made to accommodate (a) a thick overlay (often greater than 4”) and (b) nonuniformity
of the reinforcement layout. On both decks, variability in reinforcement layout was a critical
analysis consideration.
Observations made during the analysis process suggest modification to a dual-polarization analysis
approach needs to be considered in special cases, particularly when both (a) longitudinal bars are tied on
top of transverse bars AND (b) longitudinal bar spacing is either fairly large (12” or greater) or very
small (less than 4” between bars). Other considerations will also be discussed.
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Application Of A Combined Nondestructive Evaluation Approach To Detecting Subgrade Voids Below A Dam Spillway
Authors David A. Hollema and Larry D. OlsonThe combination of Ground Penetrating Radar (GPR) and Slab Impulse Response (Slab IR)
nondestructive evaluation (NDE) methods can be used to improve detection of subgrade voids beneath
reinforced concrete slabs. Subgrade voids are typically created by the eroding action of flowing water
because of an elevation or otherwise caused pressure gradient. Voids promote cracking and potential for
major long-term damage resulting in expensive repairs or replacement. These methods were recently
utilized at an alpine dam spillway for detection of potential subgrade voids.
3-Dimensional (3-D) GPR data were analyzed in a “bright spot” fashion by looking for zones
where the slab bottom reflection was abnormally strong. The cause of this data signature was attributed
to a strong contrast in dielectric between that of wet concrete and that of air or water-filled voids. Slab
IR is a model vibration test method and measures the amplitude and frequency of the slab vibration
when impacted with an instrumented hammer. Slab areas overlying voids respond with higher
amplitudes and lower stiffnesses when impacted. The 3-D GPR and contoured Slab IR results correlated
well and the same anomalous areas were discovered in each data set. Coring and video borescope were
used for confirmation of NDE delineated voids and proved the success of the methods.
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Leonardo Da Vinci’S Channel System Of Milan: Preliminary Tests On The Side Walls For The Restoration Design Project
Authors P. Ronca, Giovanni Cascante, P. Crespi and G. FranchiThe network of channels called “Navigli” was mainly built in the surrounding country and
downtown Milan during the XVI century, according to the hydraulic and engineering designs made
by Leonardo da Vinci. Many of the main streets in downtown Milan lie nowadays on these
preexisting urban channels, some of them were covered during the first half of the XX century. The
current civic administration is facing the problem of saving what remains of these complex hydraulic
and historic structures.
This paper, after a short presentation and description of the Navigli channels, describes the
preliminary in-situ tests, as well as the calibration procedure for non-destructive sonic tests
performed in order to understand the actual hidden geometry and the material condition of the
remaining of this ancient masonry wall.
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Seismic Investigation Of A Sinkhole On Clearwater Dam
Authors Richard D. Miller, Julian Ivanov, Steve Hartung and Lisa BlockA 10 ft wide and 10 ft deep sinkhole that catastrophically formed approximately 120 ft upstream
of the crest of Clearwater Dam in southeastern Missouri was the target of a high-resolution seismic
imaging program including reflection, surface wave analysis, and crosshole tomography. The primary
goals were to determine the general subsidence geometry and integrity of the core and help ascertain the
involvement of bedrock and native alluvium beneath this earthen dam. Reflection data from this survey
possess excellent frequency content (dominant >150 Hz) and provide high-resolution images of layers
within the pervious shell. Using tightly spaced surface wave profiles, an elongated low shear-wave
velocity chimney-like feature was delineated and interpreted to represent the root of the sinkhole. Based
on seismic, construction, drill, and borehole tracer data, a borehole geophysics program was designed to
identify fractures/joints that might provide seepage pathways. Crosshole seismic data detected a large
low-velocity zone within the bedrock and an associated low-velocity zone within the pervious fill
consistent with the surface seismic interpretations. A comprehensive appraisal of the risk this disturbed
zone represents to the overall integrity of the dam and whether it is a symptom of a larger, yet
undetermined subsurface leaching problem is being developed.
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