THG Geophysics
4280 Old Wm. Penn Hwy.
Murrysville, Pennsylvania 15668
Phone: 724-325-3996
Fax: 724-733-7901
info@geo-image.com
Recent Publications
Borehole Geophysics
Electric Log Analysis of Precambrian Igneous and metamorphic Rocks in the St. Francois Mountains, MO.
Abstract
Electric log, core, and optical
petrographic analysis of a metamorphosed and deformed
PreCambrian-aged rhyolite effusive event identified
2 litho-facies and effected characterization of
the rock deformation. Single-point resistance (SPR)
and spontaneous potential (SP) electric logs identified
the ubiquitous clay seams and localized dikes. The
igneous/metamorphic rocks had naturally elevated
gamma emissions; consequently, natural gamma (NG)
logging was useless in the identification of clay
seams. However, NG logs proved useful in the identification
of dikes, which had readings of less than 150 cps.
The SPR/SP suite also identified intrusive black
zones, characterized as martite by thin section
analysis, within the rhyolite.
Acoustic televiewer (AT) logs identified a conjugate shear fracture set with the main fracture set bearing a strike of N35oW and dip 80oNW or SE. The minor fracture set trends N35oE and dips 80oNE or SW. Thirty percent of the fractures logged are horizontal suggesting a vertically upward stress relief consistent with granite emplacement.
Porous zones within the rhyolite porphyry and contiguous with fractures appear to be the result of subsurface potassium feldspar phenocryst weathering and erosion. These zones are invisible to SPR, SP, and NG tools. Much of the feldspars within the rhyolite porphyry show weathering to kaolinite. Erosion and mobilization of the kaolinite is putatively considered to be the causative agent for the deposition of clay in the horizontal fracture sets.
Acoustic televiewer (AT) logs identified a conjugate shear fracture set with the main fracture set bearing a strike of N35oW and dip 80oNW or SE. The minor fracture set trends N35oE and dips 80oNE or SW. Thirty percent of the fractures logged are horizontal suggesting a vertically upward stress relief consistent with granite emplacement.
Porous zones within the rhyolite porphyry and contiguous with fractures appear to be the result of subsurface potassium feldspar phenocryst weathering and erosion. These zones are invisible to SPR, SP, and NG tools. Much of the feldspars within the rhyolite porphyry show weathering to kaolinite. Erosion and mobilization of the kaolinite is putatively considered to be the causative agent for the deposition of clay in the horizontal fracture sets.
Engineering Geophysics
Geophysical Applications to Solid Waste Analysis
Abstract
Case studies of regional landfills show that electromagnetic
geophysical methods can accurately and inexpensively
define boundaries and thickness of waste. Degradation
of putrescible solid waste generates conductive
leachate that can be imaged with a frequency-domain
terrain conductivity meter. Terrain conductivity
measurements can be modified through a simple algorithm
based upon native soil conductivity to produce plan
maps showing a detailed three-dimensional image
of the waste mass. Further, seismic records and
borings confirm that a linear relationship exists
between measured waste terrain conductance and thickness
of waste. Consequently, waste volume can be estimated
to within 15% of the true mass volume by employing
terrain conductivity mapping.
Seismic Reflection in Urban Settings
Abstract
Multi-stacked (MSF) seismic reflection data can
be used in an urban environment for imaging the
subsurface, since MSF data are not plagued by noise
from an active city. The MSF method consists of
a source and receiver, akin to a spot or jump correlation
survey common in the 1930’s. Walk-away tests document
that ground roll is eliminated from the near-source
receiver. Two MSF seismic reflection profiles from
2 active urban sites in the southwestern portion
of the USA show excellent resolution.
Field Applications of MASW Data
Abstract
One-dimensional multi-spectral analyses of surface
waves (MASW) are used to predict subsurface shear-wave
interval velocities. Shear wave velocities can also
extract additional velocity-related information
such as mechanical properties of soils and rocks.
In general, MASW data compare favorably to other
geophysical methods for predicting interval velocities.
Furthermore, comparisons to vertical seismic profiles
correlate well with MASW predicted shearwave interval
velocities.
Over 100 one-dimensional MASW records and 30 vertical seismic profiles in 5 states were collected and compared. Surface waves saturate the geophones (5-foot spread with 5-foot hammer offset) on bedrock and dispersion curves are difficult to evaluate. Consequently, MASW-derived shear-wave velocities are elevated above those derived from down-hole vertical seismic profile methods (VSP). MASW-derived shear-wave velocities collected in areas with a veneer to thick sequences of unconsolidated soil have predicted MASW shear-wave interval velocities that compare favorably to those derived from VSP data.
Over 100 one-dimensional MASW records and 30 vertical seismic profiles in 5 states were collected and compared. Surface waves saturate the geophones (5-foot spread with 5-foot hammer offset) on bedrock and dispersion curves are difficult to evaluate. Consequently, MASW-derived shear-wave velocities are elevated above those derived from down-hole vertical seismic profile methods (VSP). MASW-derived shear-wave velocities collected in areas with a veneer to thick sequences of unconsolidated soil have predicted MASW shear-wave interval velocities that compare favorably to those derived from VSP data.
Electrical Imaging of Triassic Fault System
Abstract
Continuous vertical electrical sounding (CVES) imaged
early Triassic normal faults on the southeastern
side of Mount Arlington Ridge, part of the northeast-southwest
trending ridges within the New Jersey Highlands
physiographic province of north-central New Jersey.
The site had been previously characterized with
a regional normal (down to the east) fault, the
Longwood Valley Fault that places undefined Proterozoic
bedrock units against the sandstone/conglomerate
Silurian Green Pond Formation. The sandstone/conglomerates
of the older Cambrian Hardyston Formation conformably
overlie the Green Pond Formation. The Hardyston
with the overlying Leithsville Formation and veneer
of Pleistocene-aged glacial deposits comprises most
of Long Valley within the study area.
CVES profiles collected normal to the Longwood Valley Fault show displacement within the Mesozoic sediments that are beneath the Pleistocene-aged glacial sediments in the contiguous valley. This previously unnamed fault system is termed the Kenvil Works Fault. Further, four or more synthetic faults exist between the Kenvil Works Fault and the Longwood Valley Fault. These faults accommodate right-rotational torsion between the two fault systems. The synthetic faults and Kenvil Works Fault System can act as a migration pathway for the movement of groundwater from the upland impermeable bedrock to the more permeable valley-fill sequences.
CVES profiles collected normal to the Longwood Valley Fault show displacement within the Mesozoic sediments that are beneath the Pleistocene-aged glacial sediments in the contiguous valley. This previously unnamed fault system is termed the Kenvil Works Fault. Further, four or more synthetic faults exist between the Kenvil Works Fault and the Longwood Valley Fault. These faults accommodate right-rotational torsion between the two fault systems. The synthetic faults and Kenvil Works Fault System can act as a migration pathway for the movement of groundwater from the upland impermeable bedrock to the more permeable valley-fill sequences.
Ground Water Purveying Using VLF
Abstract
Random drilling for commercially-productive groundwater
wells is a haphazard method within the Pennsylvanian-aged
rocks of the Appalachian Plateau Region of southwestern
Pennsylvania. These rocks have low permeability
and porosity and the average production well produces
only enough yield for homeowner use. Often these
wells are installed as an open hole to 300 feet
to insure an adequate water supply for the homeowner
since the well bore acts as a storage reservoir
during recovery and drawdown.
Three sites mapped with Very Low Frequency (VLF) methods delineated fractures with the potential to maximize bedrock production through increased fracture-induced permeability. A boring was advanced from a location at each of the three sites selected through VLF mapping. The borings penetrated fractures at the anticipated depths of between 10 and 20 meters below grade. Pump tests indicate that these three borings can produce between over 1,000l/min with little drawdown. Each of the three wells is a commercial success.
Three sites mapped with Very Low Frequency (VLF) methods delineated fractures with the potential to maximize bedrock production through increased fracture-induced permeability. A boring was advanced from a location at each of the three sites selected through VLF mapping. The borings penetrated fractures at the anticipated depths of between 10 and 20 meters below grade. Pump tests indicate that these three borings can produce between over 1,000l/min with little drawdown. Each of the three wells is a commercial success.
Geophysical Imaging Techniques as a Screening Tool
Abstract
Geophysical studies provide an inverse solution
to the resolution of environmental problems, and
are analogous to a murder mystery investigation.
In a murder investigation, we know a murder occurred,
but don’t know the cause (i.e., the murderer or
weapon). Environmental geophysics then is used to
image a site (i.e., the murder scene) and to identify
the target (i.e., the murderer and weapon). In this
way, geophysical representations of the subsurface
provide a fast and inexpensive method for understanding
the environmental problems prior to complex and
expensive intrusive methods.
Environmental geophysical tools exploit field conditions (passive) or measure induced conditions (active) and collect data in either a profiling or sounding method, with computers producing real-time images. Passive methods include measurements of acoustic, electrical potential, electromagnetic, thermal, radioactivity, gravimetric, and magnetic fields. The last three methods are the most commonly employed environmental methods.
Active or induced methods include acoustic, seismic disturbance, electric induction, and electromagnetic imaging. Seismic reflection is used on a routine basis to image deep in the earth; however, seismic methods can also be used to image the shallow subsurface. Resistivity is a method for profiling the subsurface using electric current. Electromagnetic methods (e.g., time-domain, frequency-domain) use a specific electromagnetic frequency to induce a secondary eddy field and are the most common tools used in environmental geophysical investigations. Ground penetrating radar, a pulsed electromagnetic tool, can provide detailed subsurface profiles.
Environmental geophysical tools exploit field conditions (passive) or measure induced conditions (active) and collect data in either a profiling or sounding method, with computers producing real-time images. Passive methods include measurements of acoustic, electrical potential, electromagnetic, thermal, radioactivity, gravimetric, and magnetic fields. The last three methods are the most commonly employed environmental methods.
Active or induced methods include acoustic, seismic disturbance, electric induction, and electromagnetic imaging. Seismic reflection is used on a routine basis to image deep in the earth; however, seismic methods can also be used to image the shallow subsurface. Resistivity is a method for profiling the subsurface using electric current. Electromagnetic methods (e.g., time-domain, frequency-domain) use a specific electromagnetic frequency to induce a secondary eddy field and are the most common tools used in environmental geophysical investigations. Ground penetrating radar, a pulsed electromagnetic tool, can provide detailed subsurface profiles.
Environmental Geophysics
Subsurface Imaging Using Non-Intrusive Ground Penetrating Radar
Abstract
Many Cemetery operators are faced
with limited space and a growing demand for burial
plots. Some have chosen to install mausoleums as
a mechanism to address this need. Unfortunately,
documentation of plot locations can be limited on
older portions of cemeteries, thus limiting development
of the property. Few methods other than digging
or probing can identify gravesites, except for imaging
the subsurface through non-intrusive ground penetrating
radar (GPR). Further, GPR is also quick and inexpensive,
since approximately an acre per day can be imaged.
GPR as a Sentinel Device
Abstract
Geophysical methods provide a non-destructive non-intrusive
method of subsurface investigation. Oftentimes,
however, the target is difficult to “see” due to
lack of contrast between the target and background.
This lack of contrast can be rectified through the
use of a sentinel device. A series of sentinel devices
were used in downtown Pittsburgh, Pennsylvania to
determine potential subsidence beneath a building
sidewalk. This sentinel strategy was performed to
predict future deformation of the sidewalk and to
prevent foot-traffic hazards.
Fracture Detection
VLF Surveying to Delineate Longwall Mine-Induced Fractures
Abstract
Very low frequency (VLF) surveying is an effective
method for detecting long, straight, electrically
charged conductors, and it has been used to locate
fractures, image subsurface voids, map landfill
margins, and to delineate buried conductive utilities.
High-powered military transmitters operating in
the 15-30-kHz range propagate far-field planar electromagnetic
waves that can induce secondary eddy currents in
electrically conductive linear and planar targets.
VLF meters record responses to the induced current
and, through filtering, can accurately locate linear
and steeply dipping planar subsurface anomalies.
VLF surveying is easy to use, deploy, and process.
Geophysical Applications to Groundwater Resources
Abstract
Water is a valuable resource and a necessary staple
to sustain life. Although water covers more than
75 percent of the earth's surface, potable water
is not always easily accessible. Random drilling
is frequently used to locate this resource, often
with disappointing results. Technological advancements
with geophysical instrumentation have dramatically
decreased the randomness of well placement for locating
significant sources of potable water.
Two types of geophysical surveys can assist in finding water, resistivity and VLF. A resistivity survey uses an earth resistivity meter to measure the resistance of the soil and bedrock. Geophysicists can now create a 3-dimensional structure of a groundwater reservoir from resistivity data. A VLF survey uses very low frequency electromagnetic energy to find water-bearing fractures in bedrock. Three-dimensional VLF surveys can locate large fractures for placement of production wells. VLF surveys can also be used to determine the quality of water within a fracture.
Case studies have shown that geophysical field surveys eliminate the guesswork in finding groundwater and help bring this valuable resource to within our reach.
Two types of geophysical surveys can assist in finding water, resistivity and VLF. A resistivity survey uses an earth resistivity meter to measure the resistance of the soil and bedrock. Geophysicists can now create a 3-dimensional structure of a groundwater reservoir from resistivity data. A VLF survey uses very low frequency electromagnetic energy to find water-bearing fractures in bedrock. Three-dimensional VLF surveys can locate large fractures for placement of production wells. VLF surveys can also be used to determine the quality of water within a fracture.
Case studies have shown that geophysical field surveys eliminate the guesswork in finding groundwater and help bring this valuable resource to within our reach.
Void Investigation
Imaging Mine Voids Using Continuous Vertical Electrical Sounding Methods
Abstract
Deep-mine voids are an insidious
hazard and early identification is imperative to
minimizing subsidence to roads, buildings and other
infrastructures. Continuous vertical electrical
sounding (CVES) can be a rapid and effective tool
for the detection of deep-mine voids; however, there
are inherent issues with CVES including cultural
noise, surface mining, and anomalous lithology.
CVES of a 60-year old deep mine in the Evansville, Indiana area imaged mine workings (i.e., room and pillar) and retreat mining of the coal. Subsequent drilling proved that deep mining had occurred as predicted by CVES. The 7-meter thick sandstone overlying a 140-year old mine void in the Akron, Ohio area is slowly collapsing and subsidence may adversely impact 69 residences. CVES profiles indicated that subsidence may affect only 25 residences and that only 6 residences were exposed to immediate subsidence. Deep-mining for clay in St. Marys, Pennsylvania, an unusual and unexpected deep-mined rock type, proved to be a potentially hazardous threat to a proposed subdivision. A field in western Pennsylvania was cleared for the installation of a manufacturing facility; however, CVES showed that the building footprint overlapped a highwall and deep-mined workings. Borings showed that the highwall and deep-mine workings were present and the building was redesigned to accommodate the potential hazards from subsidence.
Case Studies of Mine Voids Using Continuous Vertical Electrical Sounding Methods
Abstract
Deep-mine voids are an insidious
hazard and early identification is imperative to
minimizing subsidence to roads, bridges, buildings
and other infrastructures. Continuous vertical electrical
sounding (CVES) can be a rapid and effective tool
for the detection of deep-mine voids; however, there
are inherent issues with CVES including cultural
noise, surface mining, and anomalous lithology.
CVES imaging of a recently exposed limestone cave
in State College, Pennsylvania documents the usefulness
of CVES for the detection of subsurface anomalies.
Deep-mining for clay in St. Marys, Pennsylvania, an unusual and unexpected deep-mined rock type, proved to be a potentially hazardous threat to a proposed subdivision. The 7-meter thick sandstone overlying a 140-year old mine void in the Akron, Ohio area is slowly collapsing and subsidence may adversely impact roads and residences. CVES profiles indicated that subsidence may affect only 25 residences and that only 6 residences were exposed to immediate subsidence. A field in western Pennsylvania was cleared for the installation of a manufacturing facility; however, CVES showed that the building footprint overlapped a highwall and deep-mined workings. Borings showed that the highwall and deep-mine workings were present and the building was redesigned to accommodate the potential hazards from subsidence.
Deep-mining for clay in St. Marys, Pennsylvania, an unusual and unexpected deep-mined rock type, proved to be a potentially hazardous threat to a proposed subdivision. The 7-meter thick sandstone overlying a 140-year old mine void in the Akron, Ohio area is slowly collapsing and subsidence may adversely impact roads and residences. CVES profiles indicated that subsidence may affect only 25 residences and that only 6 residences were exposed to immediate subsidence. A field in western Pennsylvania was cleared for the installation of a manufacturing facility; however, CVES showed that the building footprint overlapped a highwall and deep-mined workings. Borings showed that the highwall and deep-mine workings were present and the building was redesigned to accommodate the potential hazards from subsidence.
Electrical Imaging of Mine Voids: Lessons Learned
Abstract
Deep-mine voids are an insidious
hazard and early identification is imperative to
minimizing subsidence to roads, bridges, buildings
and other infrastructures. Continuous vertical electrical
sounding (CVES) can be a rapid and effective tool
for the detection of deep-mine voids; however, there
are inherent issues with CVES including cultural
noise, surface mining, and anomalous lithology.
CVES imaging of a recently exposed limestone cave
in State College, Pennsylvania documents the usefulness
of CVES for the detection of subsurface anomalies.
Deep-mining for clay in St. Marys, Pennsylvania, an unusual and unexpected deep-mined rock type, proved to be a potentially hazardous threat to a proposed subdivision. The 7-meter thick sandstone overlying a 140-year old mine void in the Akron, Ohio area is slowly collapsing and subsidence may adversely impact roads and residences. CVES profiles indicated that subsidence may affect only 25 residences and that only 6 residences were exposed to immediate subsidence. A field in western Pennsylvania was cleared for the installation of a manufacturing facility; however, CVES showed that the building footprint overlapped a highwall and deep-mined workings. Borings showed that the highwall and deep-mine workings were present and the building was redesigned to accommodate the potential hazards from subsidence.
Electrical Imaging of Deep Clay-Mine Voids
Abstract
In the late 1890s through the 1920s,
deep mining of a local clay bed beneath 35 to 50
feet of Pennsylvanian-aged rock resulted in mine
voids in the St. Marys, Pennsylvania area. Recently,
long subsidence troughs have been noted locally
in this area. The history of the deep mining has
been all but lost to the local residents; however,
recent mine maps show that deep mining has occurred
near these residents.
Shallow-focused continuous vertical electric sounding profiles imaged subsurface haulways and rooms. Inverted measured apparent resistivity values indicate areas of subsurface voids presumable saturated. The deep mine map prepared in the 1920s shows an excellent correlation between the electrical imaging prediction of voids and the mapped voids.
Shallow-focused continuous vertical electric sounding profiles imaged subsurface haulways and rooms. Inverted measured apparent resistivity values indicate areas of subsurface voids presumable saturated. The deep mine map prepared in the 1920s shows an excellent correlation between the electrical imaging prediction of voids and the mapped voids.
Deep-Mine Void Detection through Electrical Imaging
Abstract
In the late 1800s. shallow subsurface
mining of a local coal bed beneath a 25-foot thick
sandstone resulted in mine voids in the Akron. Ohio area. Recently, localized subsidence in the subdivision overlying the deep mine has prompted officials to determine if the homes are at risk. Electrical imaging was selected for determining the extent of the subsurface voids.
sandstone resulted in mine voids in the Akron. Ohio area. Recently, localized subsidence in the subdivision overlying the deep mine has prompted officials to determine if the homes are at risk. Electrical imaging was selected for determining the extent of the subsurface voids.
Shallow-focused
electrical imaging profiles were collected near
approximately 70 homes.
Elevated measured apparent resistivity values indicate
areas of subsurface voids or zones of fracturing
related to adjacent voids.
An analysis of five parameters within a decision matrix indicates that roughly 25 homes have subsurface anomalies that warrant further investigation. Forty-four homes were not interpreted to have subsurface anomalies that warrant further investigation. Of the 25 homes considered at risk, 9 are considered low risk and 6 are considered high risk.
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