Elastic Waves in Layered Media, by Ewing, Jardetzky, and Press, McGraw-Hill, 1957.
Seismic Exploration Technique and Processing, by Hamid Al-Sadi, Birkhauser, 1980.
Seismic Stratigraphy, Robert Sheriff, IHRDC, 1980.
Seismic Data Analysis, Volumes 1 and 2, by Oz Yilmaz, Society of Exploration Geophysicists, 2001.
High-Resolution Refraction Seismic Data Acquisition and Interpretation, by Robert Lankston, in Proceedings of Symposium on the Applications of Geophysics to Engineering and Environmental Problems, Environmental and Engineering Geophysical Society, 1989.
Generalized Reciprocal Method of Seismic Refraction Interpretation, by Dereck Palmer, Society of Exploration Geophysicists, 1980.
Multichannel Analysis of Surface Waves, by Coon Park, et al, Geophysics, May-June 1999.
Ground Penetrating Radar, by Daniels, Institution of Engineering and Technology, 2004.
Seismic Shear Waves in Engineering, by Harold Mooney, Journal of the Geotechnical Engineering Division, August 1974.
Transient Electromagnetic Soundings for Groundwater, by David Fitterman, et al, Geophysics, April 1986.
Use of Transient Electromagnetics to Define Local Hydrogeology in Arid Alluvial Environment, by K. Taylor, et al, Geophysics, February 1992.
Electromagnetic Terrain Conductivity Measurements at Low Induction Numbers, TN-6, by Geonics Ltd., 1980
Electrical Conductivity of Soils and Rock, TN-5, by Geonics, Ltd., 1980
Applications Manual for Portable Magnetometers, S. Breiner, GeoMetrics, Inc., 1973.
Borehole Geophysics Applied to Ground-Water Investigations, by W. Scott Keys, Techniques of Water-Resources Investigations of United Geological Survey, Chapter E2, 1990
Technical Information and Links to Other Sites
This page presents technical information on the common geophysical and remote-sensing techniques AGI performs. Links to other web sites are provided for more information on the field instrumentation and data processing software. References to publications on geophysical theory and methodology are also provided with links to professional geophysical and geological society web sites.
Seismic waves are transmitted into the subsurface to undergo reflections from interfaces between geologic layering. The returning wave patterns are recorded by 2D or 3D geophone receiver arrays to develop reflection images of the subsurface. Larger-scale reflection surveys using vibroseis or explosive energy sources can be used to image reflection patterns from geologic layering well below 5,000 feet. These larger surveys record into 120 or more geophone positions setup on the ground surface spaced 10 meters or more apart. Smaller-scale, high-resolution reflection surveys typically use weight drops or shotgun shell blanks as the energy source and record into 80 or more geophone positions spaced 5 feet to 5 meters apart. These higher-resolution surveys are setup and field tested to record subsurface reflections from shallower depth intervals above 1,000 feet. AGI has performed numerous higher-resolution surveys using 60 to 130 channel recording configurations to develop profile images of alluvial and bedrock layering in the upper 700 feet. We also have conducted several larger-scale surveys for geological exploration projects using 120 plus channel recording configurations to record reflections from deeper earth layering below 3,000 feet.
The seismic data recorded from the reflection surveys undergoes a specialized sequence of data processing to prepare common mid-point (CMP) stacked, migrated seismic reflection profiles. These processing procedures are performed in house by AGI using the Vista 2D/3D or Visual_SUNT seismic data processing software systems. Seismic reflection data processing requires geologic judgment and is best performed by geophysicists having knowledge of site geologic conditions and subsurface exploration objectives.
Seistronix, Inc. Manufacturer of the EX-6 and RAS-24 distributed seismic data acquisition systems.
Gedco Developer of Vista 2D/3D seismic data processing software. This software provides all the capabilities of oil and gas exploration-scale seismic data processing, including pre-stack time migration.
W_GeoSoft Developer of Visual_SUNT seismic reflection data processing software. This software uses modules of the Seismic Unix processing system developed by the Colorado School of Mines Center for Wave Phenomena.
Seismic waves are transmitted into the subsurface to undergo "critical refraction" along higher seismic velocity interfaces according to Snell's law. The returning wave patterns are recorded by 2D geophone receiver arrays (and in special cases 3D geophone arrays) and used to pick first arrival times associated with the refracted waves. The resulting arrival times undergo data processing and modeling to prepare seismic compressional-wave velocity (Vp) images of the subsurface. AGI typically performs seismic refraction surveys of the upper 300 feet using weight drops or shotgun shell blanks as the energy source, with 24 to 48-channel geophone arrays. Refraction data sets are also routinely recorded from 100 plus channel geophone arrays setup for near-surface seismic reflection surveys.
AGI commonly uses one of two methods to generate seismic velocity profiles from refraction arrival time data. A conventional 2 to 4-layer velocity model is generated with the GREMIX™ refraction modeling software using the generalized reciprocal method (GRM) together with the intercept time method. This procedure works well for bedrock profiling and rock hardness (rippability) investigations. Refraction data sets with a higher density of subsurface coverage are processed with refraction tomography using the RAYFRACT™ software. The tomography modeling generates a color-contoured profile of seismic velocity variations with better imaging of sharper velocity changes associated with subsurface faulting. The subsurface velocity profiles generated from both methods reveal lateral velocity changes along refracting surfaces.
Intelligent Resources, Inc. Developer of RAYFRACT seismic refraction, crosshole, VSP tomography software.
Interpex, Ltd. Developer of GREMIX and IXRefraX™ seismic refraction interpretation software.
Shear-Wave Velocity Profiling
Seismic surface waves consisting primarily of Rayleigh-type waves (commonly known as "ground roll") are generated from "active" sources such as weight drop-induced vibrations or "passive" sources such as vibrations from background noise. These surface waves are recorded by 2D or 3D geophone receiver arrays and undergo data processing and modeling to generate seismic shear-wave velocity (Vs) profiles of the subsurface. One-dimensional Vs profiles of the upper 100 to 300 feet are typically generated by recording both active and passive surface waves into 2D and 3D geophone arrays with 24 to 60 plus geophone channels. Two-dimensional Vs profiles can also be generated by moving an active weight-drop energy source along a 2D geophone array consisting of 100 or more geophone channels. This type of "roll along" MASW shear-wave velocity profiling is often used by AGI together with reflection and refraction data recording which results in three separate profiles being generated along the same survey line to investigate subsurface conditions: Vs profile, Vp profile, and seismic reflection profile.
AGI typically uses the SurfSeis™ MASW data analysis software developed by the Kansas Geological Survey to process and model surface wave data. Other procedures such as refraction micro-tremor and interfermetric MASW are also available to analyze the frequency-dependent properties of the recorded Rayleigh waves. The SurfSeis procedure first separates the frequency components of these waves according to their phase velocity. Dispersion curves for fundamental-mode Rayleigh waves are then estimated for each geophone array. These dispersion curves can be manually picked by the user to avoid interference caused by body waves and non-fundamental mode Rayleigh waves. The resulting dispersion curves undergo a least-squares inverse modeling procedure to fit a 1D Vs model to the data. A 2D Vs profile of the subsurface can be generated by contouring several 1D Vs models from recordings into numerous overlapping geophone arrays positioned along a seismic survey line.
Kansas Geological Survey Developer of SurfSeis software for MASW data processing, dispersion analysis, and modeling.
Optim Developer of ReMi™ software which uses the refraction mirco-tremor method to analyze surface wave data to generate shear-wave velocity profiles.
Ground-penetrating radar transmits micro-wave frequency electromagnetic waves into the subsurface from radar antennas moved along the ground surface. A GPR data recording and processing system is used to display reflection pattern images of subsurface interfaces, underground objects, and conditions within concrete structures. Lower frequency radar antennas in the range 20 to 200-megaHertz are used to generate reflection profiles of geologic layering, groundwater, cavities, and fractures in the approximate depth interval 3 to 100 feet. Antennas in the 100 to 900-MHz range are typically used to image underground utility lines and substructures in the upper 10 feet. Higher frequency antennas in the range 400 to 2,500-MHz are used to image features such as metal reinforcement, voids, and fracturing within concrete structures.
The resolution of the GPR reflection patterns is highly dependent on the conditions of the surface the antenna is moved along, and the electrical properties of the target. Electrically conductive surfaces such as moist clayey (bentonite) soils and reinforced concrete pavement attenuate transmitted radar waves. In addition, the amplitude or strength of returning radar waves is also a function of the difference in electromagnetic impedance (relative dielectric constant) at the reflecting interface. The weaker this difference in impedance is the weaker the amplitude of the returning reflection is. The strongest reflection patterns are usually detected where the GPR antennas move across metal surfaces beneath the ground.
AGI typically records GPR reflection profiles along several paralleling survey lines, orientated in orthogonal directions. The GPR profiles are digitally recorded and processed to enhance weaker reflection patterns. The resulting reflection patterns are interpreted and mapped in plan view across the survey lines. Additional 3D processing of reflection patterns can also be performed when GPR profiles are recorded along more closely spaced survey lines. This higher density of data coverage can be used to prepare 3D GPR data volumes using the GPR-Slice™ processing software. The 3D GPR data volumes can be sliced along horizontal planes at various depth intervals to produce a succession of map view images of the radar reflection amplitude variations across the survey area. This processing can be used to provide more detailed plan view imaging of subsurface structures beneath a site.
AGI has also uses directionally-focused borehole radar antennas to profile the shape of subsurface concrete structures such as belled cassions beneath building foundations.
Survey Systems, Inc.
Geoscanners AB Manufacturer of specialized GPR antennas, including higher-power transmitter electronics, directional borehole antennas, and related antenna deployment equipment.
Seismic shear waves or compressional waves are transmitted to geophone (or hydrophone) receiver arrays positioned inside boreholes to generate subsurface velocity profiles. Crosshole surveys are used to send seismic waves from a "transmitter borehole" to one or more nearby "receiver boreholes, with the subsurface area of interest located between the boreholes. A specialized borehole hammer is typically used to transmit both shear and compressional wave vibrations from known depth intervals in the transmitter borehole. The receiver arrays in the adjacent boreholes are used to record the seismic waves transmitted from each depth interval. Downhole surveys commonly use only one borehole setup with a receiver array. These surveys record the seismic waves transmitted from the ground surface near the borehole to the downhole receiver array. A "walk-away" vertical seismic profile (VSP) is a special type of downhole survey where the seismic waves are transmitted to the downhole receiver array from positions on the ground surface with increasing offset from the borehole.
Crosshole and downhole surveys are commonly used by AGI to prepare one-dimensional profiles of shear wave velocity (Vs) and compressional wave velocity (Vp) with depth, based on travel time calculations. The RAYFRACT™ software can also be used with crosshole and VSP survey data to generate two-dimensional, tomographic images of subsurface seismic velocity variations.
Intelligent Resources, Inc. Developer of RAYFRACT seismic refraction, crosshole, and VSP tomography software.
AGI uses two different methods to generate subsurface electrical resistivity profiles: conventional direct-current electrical resistivity surveys, and transient electromagnetic (TEM) surveys. DC electrical resistivity surveys are typically used to generate 2D profiles of earth resistivity variations in the upper 300 feet. Simultaneous measurements of induced polarization are also possible with these surveys. TEM surveys are typically used to generate deeper profiles of earth resistivity. AGI has used TEM surveys to profile earth resistivity layering below 1,500 feet for groundwater basin investigations.
Direct current electrical resistivity surveys are performed by expanding arrays of current-inducing and voltage-measuring electrodes positioned in the ground surface. The measurements from various standard electrode arrays, such as the Schlumberger and dipole-dipole arrays, are used to calculate "apparent resistivity" versus electrode separation using an Ohm's law relationship. This apparent resistivity data is input into resistivity imaging software such as EarthImager™ and IX1D™ to implement inverse modeling for specific electrode arrays to generate 2D profiles of earth resistivity layering.
Transient electromagnetic (TEM) surveys or "soundings" are performed by setting up a square wire, transmitting loop on the ground surface and sending a short-duration "pulsed" electrical current into the loop at a specific repetition rate. The current flow through the loop setups up a short-duration "primary" magnetic field in the earth beneath the center of the loop. During the off time of this current pulse a "secondary" magnetic field is generated by the induced current flow in the earth according to Faraday's law of induction. Changes in the intensity of this secondary magnetic field with time induce a voltage in a receiver coil setup in the middle of the transmitter loop. The TEM recording system measures the voltage decay in the receiver coil and calculates an "apparent resistivity" versus time curve. The apparent resistivity versus time curve at a particular sounding location undergoes inverse modeling using the IX1D software to generate a 1D resistivity profile. A 2D resistivity profile of the earth is prepared by recording several TEM soundings along a survey line and contouring the 1D resistivity profiles generated from these soundings.
Advanced Geosciences, Inc. Manufacturer of multi-electrode D.C. electrical resistivity recording systems, and EarthImager resistivity imaging software.
Interpex, Ltd. Developer of IX1D software for DC resistivity, TEM and other electromagnetic data processing and modeling.
AGI uses frequency-domain, electromagnetic profiling instruments built by Geonics, Ltd. (such as the EM31-MK2 and EM34-3) to conduct subsurface electrical conductivity surveys. These instruments use electromagnetic induction to measure the "apparent" or average conductivity of the earth within various near-surface depths intervals. EM31-MK2 is used to measure the average conductivity of the upper 6 meters. The EM34-3 can be setup to measure average conductivity to a maximum depth of 60 meters. Both instruments can be walked along closely-spaced survey lines to record spatially-dense ground conductivity measurements. We have used these instruments to help delineate the edges of landfills and excavations, locate shallow groundwater occurrences, and map the orientation of underground structures. We typically record EM31 conductivity measurements together with GPR profiling of the subsurface.
We also use the Geonics EM61 metal detector to more accurately map the location of underground pipelines and metal substructures. We have used this device in a higher-power mode to detect larger metal objects to a depth of 5 meters.
The measurements made with the EM31, EM34-3, and EM61 instruments are typically downloaded to a field computer to prepare contour maps. Mapping software such as Surfer™ can be used to grid and contour the data and generate enhanced, color-modulated map view images.
Golden Software, Inc. Developer of Surfer contour and 3D surface mapping software.
A portable magnetometer or magnetic gradiometer is used to measure the magnetic field of the earth. Anomalous measurements (caused by a localized distortion of the magnetic field lines) are recorded over areas where ferrous objects are buried or sudden changes in rock magnetization occur. Measurements of the "total magnetic field" in units of "gammas" are recorded along paralleling survey lines to map the magnetic field variations across a site. AGI typically records these magnetic field measurements together with GPR profiling of the subsurface.
The total magnetic field and magnetic gradient measurements are downloaded to a field computer to prepare contour maps. Mapping software such as Surfer™ can also be used to grid and contour the data and generate enhanced, color-modulated map view images. At mid-latitude locations, such as the Western US, "diople" and "monpole"-like anomaly patterns can be recognized on contour maps to help interpret the approximate depth, shape, and orientation of subsurface objects.
GeoMetrics, Inc. Manufacturer of G856 proton precession magnetometer,G858 and G859 cesium magnetic gradiometers, and related data processing software.
Standard borehole geophysical logs such as electrical resistivity, spontaneous potential, electromagnetic induction, and natural gamma radiation logs are used to measure various physical properties of the borehole formation. A combination of several of these logs are typically recorded and used to help correlate lithologic conditions (such as alluvial stratigrahy) and identify groundwater producing intervals in boreholes.
AGI uses these logs to help correlate borehole information to seismic reflection profiles and other geophysical profiles to investigate subsurface faulting or position well screen intervals in groundwater wells.
echo testing uses vibrations generated
from a small hammer impact to evaluate the structural integrity of a concrete
pile shaft constructed in the ground. To perform this testing AGI
uses procedures that conform to ASTM D5882-7 and typical construction
contract specifications. An accelerometer is tightly mounted with
wax to the smoothed surface of the top of the pile. A small,
plastic-coated hammer is used to send a sonic pulse down the length of the pile to
reflect a pulse back from the bottom "toe" of the pile. A
digital recording of the vibrations measured by the accelerometer is
displayed to evaluate the quality of the toe reflection pattern. The
detection of a relatively strong toe reflection pattern at the expected
travel time (calculated from the known length of the pile and acoustic
velocity of concrete) indicates a uniform concrete pile shaft without
significant defects. The detection of earlier arriving pulses from acoustic impedance interfaces above the toe is an indication of
possible structural defects such as a diameter reductions (caused by soil
inclusions) or a fractured, broken areas of the pile shaft.
Pulse echo testing uses vibrations generated from a small hammer impact to evaluate the structural integrity of a concrete pile shaft constructed in the ground. To perform this testing AGI uses procedures that conform to ASTM D5882-7 and typical construction contract specifications. An accelerometer is tightly mounted with wax to the smoothed surface of the top of the pile. A small, plastic-coated hammer is used to send a sonic pulse down the length of the pile to reflect a pulse back from the bottom "toe" of the pile. A digital recording of the vibrations measured by the accelerometer is displayed to evaluate the quality of the toe reflection pattern. The detection of a relatively strong toe reflection pattern at the expected travel time (calculated from the known length of the pile and acoustic velocity of concrete) indicates a uniform concrete pile shaft without significant defects. The detection of earlier arriving pulses from acoustic impedance interfaces above the toe is an indication of possible structural defects such as a diameter reductions (caused by soil inclusions) or a fractured, broken areas of the pile shaft.
Pulse echo testing is typically performed during construction operations, immediately after the concrete has reached its design strength and before pile caps, grade beams, and other foundation elements are attached. AGI has performed this testing for several large construction projects for cast-in-drill-hole piles, auger piles, and pre-cast piles associated with earth structures such as bridges, building foundations, and transportation corridors.
Other higher-frequency sonic and seismic methods are also used to evaluate the conditions of subsurface foundations. Downhole refraction measurements from adjacent boreholes can be used to investigate the length of larger diameter pile shafts or subsurface walls attached to buildings. Crosshole measurements from two or more pairs of boreholes can also be used to evaluate the structural integrity of larger diameter piles and concrete structures positioned between the boreholes.