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13th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 20 Feb 2000 - 24 Feb 2000
- Location: Arlington, Virginia, USA
- Published: 20 February 2000
121 - 140 of 140 results
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Contribution Of Electrical Resistivity Methods To Paleoenvironmental Reconstruction And Groundwater Exploration In The Chihuahua Desert, Mexico
Authors J.M. Maillol, J. Ortega-Ramirez, W.L. Bandy and A. Valiente-BanuetIn Mexico, arid and semi-arid areas represent more than 50% of the country’s territory, and
drought and desertilication represent major challenges to sustainable development. This has
prompted an interdisciplinary study of the Quaternary evolution of the El Fresnal valley in the
Chihuahua Desert (Northern Mexico). The main goal is to confirm evidence suggesting the
existence of less arid episodes during the late Quaternary, which could have important
consequence for the availability of groundwater in the area. Direct detection and characterization
of potential aquifers is also addressed.
Electrical resistivity surveying is used to help reconstruct the geometry of sedimentary deposits,
in particular alluvial fans. Classical 1-D soundings distributed along profiles spanning several
kilometers help unravel the geometry of the deposits. 2-D resistivity imaging is used to
investigate smaller scale features. Data inversion produces resistivity images that apparently
reveal details of the deposits, but as always it is important to assess the reliability of the
inversions.
Tests based on the inversion of two intersecting profiles are conducted using two distinct
inversion methods. Comparisons of the results are encouraging and show generally good
agreement between 1) different methods and 2) intersecting sections. Nonetheless, the
comparatively minor discrepancies observed serve as a reminder that imaging techniques must
be used with proper caution.
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Modulated Active Magnetic Surveys For Sub-Surface Utility Mapping
More LessA quasi-static magnetic method has been developed to indicate the exact location of sub-surface
utilities. Standard radio frequency EM transmitter methods are complicated by distortions
associated with induction in secondary targets including underground services and surface
metallic artefacts. Similarly, standard high definition magnetic methods are subject to high
ambient noise levels and ambiguities associated with superposition of responses from multiple
sources. The modulated active magnetic system (MODAM) has now been developed to avoid
these complications. Static magnetic fields are readily removed from the total field response by
comparing on-time and off-time magnitudes, allowing spatial vectors (defining the transmitter
location) to be resolved with great precision. Results obtained from MODAM field trials have
been confnmed with electronic total station surveys, giving an agreement better than 0.05 m at
12 m depth.
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Combining Surface And Borehole Geophysical Techniques To Locate And Define A Buried Outwash Aquifer In Central Illinois
Authors Timothy C. Young and Timothy H. LarsonSand and gravel deposits comprise the primary aquifers supplying water to most small communities
in east-central Illinois. In eastern Champaign County, a few thick narrow bands of outwash, 600 to 1,200
feet wide, were deposited in bedrock channels or trailing away from receding glacial fronts. These narrow
deposits, now buried by 50 to 130 feet of younger drift, are difficult and impractical to locate using test
drilling as the sole means of exploration. However, because the outwash deposits are encased within clayey
glacial till and overlie shale bedrock, they are excellent targets for electrical earth resistivity (EER) surveys.
Downhole geophysical methods are very sensitive to the minor variations in sand and clay content that signal
facies changes in diamicton and outwash. When used together, surface and borehole geophysical methods
provide a powerful means for locating, confirming and delineating sand and gravel aquifers as well as aiding
in optimum construction of water supply wells in this hydrogeologic setting.
Following 10 years of attempting to locate an aquifer through periodic test drilling, the Village of
Homer and Sodemann & Associates Engineering of Champaign contacted the ISGS for assistance. Guided
by a Groundwater Possibilities Report written to the Village of Homer in 1992 by ISGS Geologist Ross
Brower, a site was selected and an EER survey was conducted in June, 1996, 2 to 5 miles north of the
Village of Homer, Illinois, to delineate a buried outwash deposit. To locate the narrow band of outwash,
resistivity stations were located about 500 feet apart. Using a prominent resistivity anomaly as a guide, test
drilling at three sites confirmed the presence of a lOO-foot thick sand and gravel deposit overlain by 40 to
50 feet of glacial drift within an area known for having limited groundwater potential, even for rural
domestic supplies. Rotary drill cuttings and borehole geophysical logs specifically indicated a horizontal
facies change within the deposit and a narrow bedrock channel where the thickest outwash (100 feet thick)
was deposited. Thin outwash sand and gravel deposits, normally separated by clay layers, merged into a
single thick deposit that is the aquifer in which the new village’s wells were completed.
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A Field Test Of The Induced Polarization Method In Aarhus County, Denmark
Authors Stewart K. Sandberg, Lee Slater, Kurt Sørensen and Esben AukenFor several years, DC resistivity and transient electromagnetic methods have been
successfUlly employed in Denmark to map variations in lithology and thicknesses of surficial
sediments. Research initiatives have brought about significant improvements in data
collection efficiency, which have in turn allowed an unprecedented spatial density of data to
be obtained. Mapping electrical conductivity by these methods has been shown to provide
the necessary resolution of near-surface lithology for delineating both surficial aquifers and
aquicludes, based on their conductivity differences. To further this mapping effort, we
investigated the use of induced polarization in delineating hydrogeological boundaries and
hydraulic properties within the sediments and bedrock comprising the surficial aquifer
system in Aarhus County, Denmark. A field test of the induced polarization method
occurred in Aarhus County, Denmark during the period from May 10 - 2 1, 1999. Field
locations were selected, based on representative geological significance, and where the
hydrogeology was well known (based on pre-existing borings, lithological logs, water
chemistry, geophysical well logs of these borings, and other surface geophysical data).
Resistivity and induced polarization soundings were obtained at a total of five field areas, 1)
the Aarhus geophysical test area, 2) Beder, 3) Longballe, 4) Olst, and 5) Grenaa. In addition,
laboratory measurements of the spectral induced polarization response of some of the
lithologic units were performed.
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Use Of Anderson-Style Linear Digital Filters For Transient Electrical Fields Calculation
More LessThe interpretation of marine transient electrical measurements requires fast methods of
calculation of time-domain fields. It is known that digital linear filters could efficiently solve the
task. This paper describes how it is possible to design such filters for &4,M,B or ABM,M,
array. SVD technique was used to design them. These filters allow fulfilling calculation in the
presence of induced polarization.
These filters were very useful in our routine modeling and in the interpretation of field
data, which was received in the Barents Sea during oil and gas prospecting. The filters may be
also used for taking into account induction in ground DC and IP measurements with linear
arrays.
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2-D Electrical Resistivity In A Complex Geological Environment
Authors Clyde A. Ringstad, Matthew C. Ringstad and Lynn M. RingstadConstruction of a new office park / housing development East of Seattle, adjacent to the
l-90 corridor, requires construction of a new access road, the proposed South
Sammamish Plateau Access Road (SSPAR). The steep slopes adjacent to the
interstate highway were identified as a possible deep-seated ancient landslide.
Initially, numerous borings were completed with somewhat ambiguous results with
regard to delineating the presence of a slide. Seismic Refraction lines and
Electromagnetic profiles were also initially completed at the site by others with similar
results.
Based upon our 2-D Electrical Resistivity data, the site interpretation was indicative of
very complex geology, which was further substantiated with new boring data. The
subsurface geology appears to range from a mixture of clayey silt deposits up to large
ice rafted boulders, the size of a house. The primary conclusion drawn from the 2-D
Electrical Resistivity data defined what appear to be subsurface channels cut roughly
parallel to the steep site slopes. These channels are thought to be erosional from
meltwater flows of alpine glaciers. They may also result from the catastrophic draining
of a large glacial lake just East of Seattle. The channels were subsequently infilled with
soils derived from nearby deposits, causing the difficulty in both visual and earlier
geophysical exploration.
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Magnetic Data Interpretation In A High Noise Urban Environment
Authors Jennifer Holt and Jeffrey DanielsGeophysics is increasingly being used in high noise urban environments,
like Brownfields. Ground penetrating radar (GPR) or induction electromagnetic
methods (EM) are the most widely used technique for site characterization in
these settings since it is generally assumed that utilities and buildings make it
difficult to obtain good magnetic measurements. However, recent tests at a
Brownfields site in Chicago show magnetic methods can also be used in an
urban environment. The vertical gradient of the magnetic field is most often
used, but at this site the horizontal gradients added significant information to the
interpretation. In addition, the three different gradient components show
alternate trends of the anomalies. Many subsurface features, like pipes, run
north-south or east-west in urban areas, and it is important to take measurement
in each direction to improve the likelihood of finding such features. It is possible
to combine the two horizontal gradient components to improve the interpretation
by removing some of the bias introduced by running lines in a particular direction.
Finally, there is the importance of contour scale. Different interpretations can be
made with different contour intervals making the scale a very important part of
the interpretation. Taking all this into account makes magnetic measurements a
viable option in a high noise urban environment.
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High Resolution Spectral Gamma Logging At The Hanford Tank Farms
Authors R.G. McCain, C.J. Koizumi and J.F. BertschBaseline characterization of vadose zone contamination at the Hanford Tank Farms is being
performed by the U.S. Department of Energy Grand Junction Office (DOE-GJO). This project
involves detection and assessment of gamma-emitting radionuclides through cased holes. This is
the first step toward developing an understanding of the nature and extent of subsurface
radioactive contamination associated with past tank leaks. The equipment and methods used to
establish a credible and defensible baseline data set are discussed, and significant findings are
presented. Naturally occurring 40K, 238U and 232Th were quantified, as well as man-made
radionuclides, primarily 137Cs,6 oCo, 152”54E~a, nd 235’238UT. he data refute long-held assumptions
that contamination associated with tank leaks did not migrate significantly.
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Gamma Radioactivity Differentiation Of Weathered Volcanic Tuff In Hong Kong
Authors Qinfang Chen and Lung S. ChanA study of the relationship between weathering and gamma radioactivity of rocks has been
conducted in Hong Kong to characterize and delineate subsurface clay-rich zones that are
potential landslide slip planes. Measurement results of major element contents in 19 volcanic
tuff samples reveal a progressive decrease in potassium content from fresh to weathered
rocks, while uranium and thorium contents varied in a non-systematic manner. The more
weathered rocks also showed a reduced gamma emission associated with potassium as well as
a lower total gamma emission. Both the K content and the gamma emission from potassium
show a strong correlation with the chemical weathering index, suggesting the potential use of
the gamma spectroscopy as a suitable indicator to the degree of weathering in clay-rich
material. This finding provides empirical support for the application of downhole radiometric
survey in the process of geological ground investigation.
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Comparison Of Em Systems At The Western Michigan University Geophysical Test Site
Authors Justin M. Bailey and William A. SauckThe responses of three different EM systems were measured and compared over
the WMU geophysical test site in Kalamazoo, MI. Detailed contour maps of the 40m x
200m area were made of the in-phase and quadrature readings for each frequency, and for
two delay times for the time domain system. The systems included the EM3 l-MK2,
GEM-300, and EM61-HH. The WMU Geophysical Test Site is 50m x 200m in area and
has steel pipes, 55-gallon stell drums, and plastic drums buried singly and in combination
at various orientations and depths. The site was surveyed with lines one meter apart for
each instrument. Each system has its advantages and disadvantages, related to the use it
was designed for. Some are more sensitive to certain types and sizes of targets than
others. The GEM-300 was sensitive but very drift-prone and subject to large offsets from
one line to the next. On-line gradients and the line-to line shifts could be removed with
some effort. The EM3 1 revealed some interesting polarity reversals related to depth of
burial of the drum or to the transition from single to multiple drums.
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Application Of Geographic Information Systems (Gis) To The Management And Evaluation Of Remedial Investigation Data: A Case Study
Authors A. Edward Sciulli and William E. StengerThe management of Remedial Investigation (RI) data has historically been accomplished using
standard spreadsheets and manual input and manipulation processes. Scientists and engineers,
who are under ever increasing pressure to produce more for less, have long been constrained by
these inherently inefficient and often inaccurate methods. The advent of desktop GIS has placed
powerful data management and interpretation tools into the hands of scientists and engineers.
Now, projects and schedules are less affected by the limited availability of data processing
personnel, computer aided drafting (CAD) operators, or plotters. Real-time evaluations can be
conducted saving both time and resources. This paper presents a case history of a project which
used GIS to manage the collected data. The New Jersey Department of Environmental
Protection (NJDEP) conducted a comprehensive RI of a former industrial site in southern New
Jersey. The RI generated over 55,000 records of analytical and other data requiring evaluation
and interpretation. A relational database and GIS, designed specifically for remedial
investigation data was used. This technique reduced data management time, provided scientists
and engineers the resources needed to conduct more thorough evaluations, and generated the
visual presentation items. Based on the results of the RI, the site is currently under consideration
for inclusion on the USEPA National Priority List (NPL).
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Multifrequency Em Mapping For Improved Site Characterization: Case Histories
Authors Dean Keiswetter and I.J. WonElectromagnetic methods can be used to effectively and economically identify and locate
subsurface features. To satisfy most environmental project objectives, the selected
method must possess minimal field logistics, rapid survey speed, and produce high
quality information. Geophex has designed a logistically simple, multifrequency
electromagnetic induction (EMI) sensor that is capable of operating over a bandwidth of
30 Hz to 24 kHz. Although it is widely recognized that buried objects produce a
frequency dependent response, case histories illustrating the need for broadband EM1
data for site characterization projects are scarce. We present selected case histories that
demonstrate the importance of measuring a broadband EM1 response during
environmental site characterization projects.
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Viscoelastic Finite-Difference Modeling With Application To Shallow Seismic Refraction Data
More LessI have developed a two-dimensional, viscoelastic, finite-difference modeling method for
complex surface topography and subsurface geological structure. Realistic modeling of seismic
wave -propagation in the near surface is complicated by many factors, such as strong
heterogeneity, topographic relief and large attenuation. In order to account for these
complications, I use a velocity-stress staggered grid and employ an 0(2,4) accurate viscoelastic
finite-difference scheme. The implementation includes an irregular free surface condition for
topographic relief. The algorithm is applied to 2-D modeling of the viscoelastic response of near
surface structure beneath a 2-D refraction survey line. The P-velocity models were constructed by
2-D traveltime tomography, and S-velocity, density and Q were given empirically. Comparison of
the observed waveform data with viscoelastic response clearly demonstrates the importance of
inclusion of viscoelasticity. The character of the observed waveform data can be explained by
velocities and Q distributions.
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Reconstruction Of Near-Surface Fracture Areas By Means Of 3D Focusing Transformations
Authors V.A. Pozdniakov, D.V. Safonov, V.A. Tcheverda and T. TushkoOur experience of 2D seismic data processing (both synthetic and real) by means of
focusing transformations approved the possibility to stress some specific features of the
geological medium under consideration, especially such as fractured and cracked areas. The
next step naturally should be 3D data processing of the real field data. The paper presents
results those were accomplished by processing of seismic data those were gained on the testsite
placed on the south of the Krasnoyarsk area. This test-site is in action for a recent 3 years
and is studied rather well by the range of geophysical methods (seismics, gravies,
magnetotelluric and some others). The most significant feature of this test-site is the nearsurface
presence of bazalt intrusion followed by the rather complicated upper part of the
cross-section.
The interior of this intrusion is not homogeneous itself and possesses a lot of fractures
and cracks. The very important question for a range of geological problems is to map
correctly the spatial disposition of these areas (for example when one deals with the problem
of industrial waste disposal). The results presented in the paper demonstrate the possibility to
do this by means of 3D focusing transformation. The specific fracture was spatially followed
and traced for hundreds meters. It should be noted that initially this fracture was revealed by
means of 2D focusing transformation.
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Construction Of 2-D Vertical Shear-Wave Velocity Field By The Multichannel Analysis Of Surface Wave Technique
Authors Jianghai Xia, Richard D. Miller, Choon B. Park and Julian IvanovWe present a method that utilizes the Multichannel Analysis of Surface Waves (MASW)
technique and a standard common depth point (CDP) roll-along acquisition format similar to
conventional petroleum exploration seismic data acquisition to construct a vertical section of the
near-surface shear (S)-wave velocity field. A one-dimensional (1-D) S-wave velocity vs. depth
plot is obtained by inverting phase velocities using the MASW technique. This 1-D profile
appears to be most representative of the materials directly below the middle of a geophone
spread. Multiple 1-D plots of S-wave velocity vs. depth are generated as the source and receivers
roll along a survey line. A two-dimensional (Z-D) vertical cross-section of S-wave velocity can
be constructed by contouring grids produced by combining all the 1-D S-wave velocity profiles
that are a function of the middle point of geophone spread (x) and depth (z). The combination of
inverting the phase velocity for S-wave velocity and the standard CDP roll-along acquisition
format makes this a very effective and time-efficient method of imaging two-dimensional Swave
velocity along a survey line. There are several advantages that make this method attractive
in real world applications. 1. The method focuses on high-frequency (2 2 Hz) ground roll to
provide a 2-D near-surface S-wave velocity map and to detect targets significantly shallower
than feasible with other acoustic techniques. 2. That ground roll, which is acquired by the multichannel
acquisition method, has a high signal-to-noise ratio, allowing 2-D images to be obtained
in extremely noisy environments. 3. The method uses the standard CDP roll-along acquisition
format, which provides an efficient way to acquire large quantities of broadband surface wave
data along a line. 4. The method utilizes the redundancy of the standard CDP roll-along
acquisition format so that it not only provides a reliable way to verify inverted S-wave velocities,
it also reduces the ambiguity of inverted S-wave velocities. 5. A 2-D display of S-wave velocity
can be produced easily and quickly by contouring the inverted S-wave velocity to provide a map
of the S-wave velocity field. 6. 2-D data processing techniques, such as regression analysis,
could easily be applied to a vertical S-wave velocity section to enhance local anomalies (gas or
oil fields, voids, tunnels, etc.). More than five thousand shots of MASW data have been acquired
and processed producing more than forty vertical near-surface S-wave velocity sections since
1997. Four real world examples demonstrate the usage of the method.
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Collecting Seismic-Reflection Data From Depths Shallower Than Three Meters
Authors G.S. Baker, D.W. Steeples, C. Schmeissneti and K.T. SpikesWithin the last three years, the ability to
describe the upper three meters of the earth’s
subsurface seismically has advanced significantly.
One reason is a better understanding of the
requirements for successfully collecting data at
these shallow depths. First, geophones placed in
close proximity to each other do not produce
significant mechanical or electrical crosstalk, and
high-quality seismic-reflection data have been
collected at several locations with geophone
intervals as small as 5 cm. This high-density
coverage of the near-source wavefield has allowed
the identification of ultrashallow (less than 3-m
deep) reflection events that otherwise would not
have been visible. Second, previous notions about
seismic sources, specifically when they concern
information contained in the near-source
wavefield, have been adjusted. For example, nearsource
waves are possibily distorted by nonlinear
displacements at the source. By greatly reducing
source energy, identifying reflection and
refraction events at frequencies of 500 to 1000
Hz within a few meters of the source becomes
possible. Third, an increased understanding of
the effects of near-surface moisture conditions on
data quality has been gained. Data collected over
time at a site with all things equal except for
variable surface-moisture conditions have yielded
results with widely varying quality.
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Surface-To-Tunnel Seismic Tomography On Kilometer Scale At Yucca Mountain
Authors T.M. Daley, R. Gritto, V.A. Korneev, M.A. Feighner, E.L. Majer and J.E. PetersonA surface-to-tunnel seismic survey was conducted to estimate fracture intensity and distribution in
the potential nuclear waste repository at Yucca Mountain, Nevada. We located 161 source positions
(approximately 5 km at 30-m spacing) and 224 receiver positions (approximately 3 km at
15-m spacing) on top of Yucca Mountain Ridge and inside the ESF tunnel, respectively. Zerooffset
amplitude variations of the first arrivals at the receiver line inside the tunnel showed a high
correlation with the fracture density as determined by measurements along the tunnel wall.
Analysis techniques were developed to allow 2D tomographic imaging (both velocity and attenuation)
of the horizon of interest. Tomographic inversion of the data revealed a strong correlation
with fracture-mapping results along the ESF. The velocity tomography results were then converted
to fracture-density tomograms. The results show the location and distribution of fractures
over a large area of the potential repository site.
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Detailed Structural Survey Of The Shallow Part Of A Buried Active Fault Using Highresolution S-Wave Reflection And Seismic Cone Penetration Tests
Authors Tomio Inazaki and Naomi KanoWe conducted a high-resolution S-wave reflection survey using Land Streamer across a
buried part of an active fault at the western edge of the Niigata Plain, central Japan. The
survey line was set on a paved country road. The Land Streamer, which is featured by the
towing belts on which geophone units are mounted, provides high-quality data even at the
paved areas in spite of its soft coupling to the surface. However, it is hard to estimate the Swave
velocity of the surficial layer beneath the pavement, because of being masked by the
pavement of high velocity. We therefore carried out the seismic cone penetration tests
(SCPT) to clarify Vs profiles of the near surface. By means of the SCPT data, we
successfully determined the depths of reflectors. The reflection profile clearly delineates a
detailed structure of the surfrcial layers to a depth shallower than 60 m, and showed a
deformed structure in the Holocene layers, which is strongly inferred to be formed by
faulting of the existing active fault.
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Characterization Of Near-Surface Soil And Rock Properties From Shallow Seismic Observations
Authors S.S. Alexander, D.M. Falkenstern, T.J. Gebbie and M. ZeisloftP, S, and Rayleigh waves from shallow seismic observations have been used for several
environmental applications in central Pennsylvania. Soil properties and bedrock fracturing
distributions were measured in two watersheds with shale bedrock and one watershed with
limestone bedrock. Empirical relationships from in situ measurements were used to relate Pvelocity
to fracture density and hydraulic permeability in the shale bedrock areas. Discrete,
more-widely-spaced fractures extending to significant depths characterize the limestone bedrock,
whereas the shale fracture density decreases rapidly with depth. These bedrock fracture
distributions control the shallow subsurface flow of groundwater and are important for mitigation
of non-point-source groundwater pollution from human activities such as farming. Related
studies are being carried out by the U.S. Department of Agriculture to assess the effectiveness of
riparian vegetation in reducing the concentrations of nitrates that enter the streams from nearby
farms. These seismic results are used to constrain flow models and estimate the proportion of the
flow near a stream that can interact with the root systems of the riparian vegetation. Significant
lateral variability in soil properties and bedrock fracturing was observed in both the shale and
limestone settings. The data also show that first-, second- and third-order stream locations are not
controlled by fault-related fractures in the shale bedrock. At another site a suite of shallow
seismic profiles was used to determine the thickness of mine spoil and look for localized
fractured zones in the mine floor of an abandoned coal strip mine to help determine its suitability
for use as a landfill. In all of these applications Rayleigh wave dispersion observations provide
spatial resolution of soil and bedrock properties on a scale of a few meters that supplement the
refraction and reflection interpretations.
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Shallow Seismic Reflection Survey At Wickiup Dam In Central Oregon
Authors Richard D. Miller and Richard D. MarkiewiczConcerns about potential liquefaction and resulting flowslide type failures at Wickiup
Dam in Central Oregon prompted geophysical studies of the left wing dike, studies which included
a high resolution seismic reflection survey. High resolution seismic reflection data effectively
delineated structures and correlated lithologies to shallow borehole data above the surface of the
dipping, basalt bedrock on the downstream side of the dam. Unconsolidated, Quaternary lacusnine,
fluvial, and volcanic-derived sediments overlay the bedrock surface, which is encountered
within 50 feet of the ground surface near the dam outlet works and plunges to depths of at least
1000 fi below ground surface 5000 ft west of the outlet works. Sediments that filled the seismically
defined bedrock surface possess a wide range of bedding geometries likely indicative of
multiple episodes of erosion and deposition, driven by the modern and ancestral Deschutes River
system. Dominant frequencies in excess of 250 Hz evident on stacked sections provide vertical
bed resolution on the order of 10 ft at depths of over 900 ft. Layering interpreted on stacked
seismic data within the basaltic bedrock has been inferred to represent contacts between different
basalt flow episodes and volcanic sediments (ash, cinder, etc.) originating from Wickiup Butte.
Frequency inversions, which are common indicators of inappropriately stackedinterpreted
seismic sections, evident on these stacked data are actual inversions that can be verified
by correlating the stacked sections with shot gathers, a process rarely possible on high resolution
stacked seismic sections. If geologic interpretations, instrumental in engineering designs for
resistance to seismic loading, would have been based on existing drill data from the upper 300 ft
alone, a relatively flat laying, cyclic sequence of sands, gravels, volcanic ash, silt, and clay overlying
the mudflow and basalt materials would have been inferred to an estimated depth of several
hundred to more than one thousand feet. Seismic data provided an extremely detailed image of
the subsurface, delineating channel features within the sediments and a dipping basalt bedrock
surface. Amplification potential of these kinds of geometries may impact the levels of expected
ground motion for a given size seismic event.
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