• Advances in Subsurface Modelling Using Seismic Refraction Data

  • INEL Multicomponent NanoTEM data

  • Improved near surface mapping in groundwater studies: Application of fast sampling time domain EM su

  • Zonge Electrical Tomography Acquisition System

  • The Inversion of Magnetotelluric Data and the Elimination of Topographic Effects Through Modelling

  • Geophysics as an integral part of the Aquifer Storage and Recovery (ASR) process

  • The effect of electrode contact resistance on electrical field measurements

  • Case Histories of Buried Borehole Detection: An Exercise in Flexibility

Advances in Subsurface Modelling Using Seismic Refraction Data

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Standard geophysical survey practice using seismic refraction techniques has predominantly produced two-dimensional cross sections of the subsurface. The state-of-the-practice for nearly three decades has been to process refraction data with layer reconstruction techniques using the generalized reciprocal method, time-intercept and other similar techniques. Within the past decade, advancements in the computer technology and the development of tomographic modelling algorithms have greatly increased the ability to detect subsurface anomalous features, increase lateral and vertical resolution, and provide better graphical presentation of the data. Recently, 2D finite-element modelling of seismic data has proven successful to image discrete anomalies such as voids.

This paper presents recent developments in a new approach for processing refraction data, the presentation of subsurface data, and the use of these data after geophysical modelling is complete. The approach adapts numerical modelling using the discrete element method and particle flow code (DEMPFC). The procedure is termed the Geostructural Analysis Package (GAP) which, in its initial stages of development has been optimized for geotechnical applications, such as 2D and 3D seismic refraction data processing and presentation on engineering projects. Although GAP has not been primarily created for seismic refraction, this paper will illustrate significant advancement in refraction data processing. Currently, using GAP for seismic applications represents an innovative approach that includes improved data analysis processes and produces more functional result for the end users. For the application illustrated in this paper the end users are typically civil or geotechnical engineers. The value of using this approach for seismic applications is its ability to produce 2D, 2.5D and 3D models to assist engineers or geologists extract additional information from the geophysical data (e.g., material properties), or perform static and dynamic stress analysis. This paper makes the point that mapping the top of bedrock may be the objective of a geophysical survey, but it is not the engineering purpose for the site investigation (e.g., construction of a critical facility, design of a foundation for a structure, etc.). With high-quality calibrated 2D, 2.5D and 3D DEM-PFC models, not geophysical images, engineers are more likely to use the seismic results by incorporating them directly into their engineering analyses.

Results from two case histories are presented showing the benefit of assessing seismic refraction data using the DEM-PFC numerical modelling approach. In the first example, standard 2D refraction data were analyzed and the interpolated results were presented as a 3D model. The second example is a 3D surface tomography reconstruction of four slightly offset 2D refraction shot lines.

 

INEL Multicomponent NanoTEM data

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A new enhancement to the GDP-32 receiver permits it to gather NanoTEM data on three channels simultaneously. This multi-channel capability was used recently on a research project to acquire all three components of the magnetic field (Hx, Hy, and Hz) at each station on a grid over an environmental test pit in Idaho. The work was done at the Idaho National Engineering Laboratory (I.N.E.L.) as part of the Electromagnetic Integrated Demonstration (E.M.I.D.) project.

Improved near surface mapping in groundwater studies: Application of fast sampling time domain EM surveying methods

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There are a limited number of methods currently in use to gather information on the various hydrogeological/environmental problems that are part of "everyday" life. Traditionally, groundwater problems have been evaluated and then monitored using a carefully designed network of wells where water depth and quality are measured on a regular basis. In recent years, some of the various mining and petroleum oriented geophysical techniques have been modified from their deeper applications to sample at shallower depths. Often the goal of these surveys is to help geologists and engineers determine whether their assumptions on well location and water flow are correct. Techniques that have been used include shallow seismics, DC resistivity, ground penetrating radar, frequency domain electromagnetics (FDEM), and time domain electromagnetics (TEM).

Recent advances in sampling speed, circuitry speed, and data recording have allowed the development of TEM techniques where data can be taken faster (and therefore start closer to the surface), and with better resolution of the top 15-50 m. These techniques include the Zonge Engineering NanoTEM system and the fast sampling modifications to the SIROTEM-3 system.

This paper briefly summarises the TEM results from three separate study areas encompassing a range of hydrogeological and environmental problems, each of some immediate importance in Australia at this time. The first study, at the Stockyard Plain Disposal Basin (SPDB) near Waikerie, South Australia, examines the changing hydrological environment around a groundwater disposal basin in the Murray-Darling system. The second study, in the Willaura Catchment in Victoria, examines water mobility in an evolving dryland salinity system. The third study, at an abandoned mine site in New South Wales, attempts to delineate the extent of acid-mine drainage in the area around the mine.

Zonge Electrical Tomography Acquisition System

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For a number of years, Zonge International has been involved in the development of instruments for electrical resistance tomography (ERT) in connection with ongoing research and development at the Lawrence Livermore National Laboratory, a facility funded by the U.S. Department of Energy. ERT is being used to generate resistivity images of the 'plane' defined by the space between two boreholes or other electrode strings. ERT is presently being applied to monitor leakage at hazardous waste sites, to monitor dynamic fluid injection processes that are often involved in waste site remediation. Other applications for ERT technology exist and are simply waiting for the availability of equipment and software that can economically acquire the necessary resistivity data required and the software required for its interpretation.

ERT involves the acquisition of hundreds, even thousands of 4-electrode resistivity measurements that are possible between multiple strings of electrodes. For example, given two strings of 15 equally-spaced electrodes (30 electrodes total), there are 632 different dipole-dipole measurements that can be made (including all reciprocal measurements) involving transmitter and receiver dipoles with a fixed length of 2 electrode spacings. The dense data provide the basis for solving sophisticated inverse computer models of the conductivity distribution in the ground. When both resistivity and IP are measured, the resulting data set relates to the complex impedance of the earth and the term EIT (Electrical Impedance Tomography) is sometimes used to describe the technique.

Obviously, there are too many measurements to be acquired manually. To efficiently measure all the desired transmitter-receiver electrode combinations requires a computerized acquisition system that automatically switches both transmitter and receiver electrodes and has multi-channel measurement capabilities. The Zonge ERT/EIT acquisition system has unique capabilities for the efficient acquisition of ERT or EIT data sets. With the Zonge ERT/EIT system, acquisition of the dense data sets is now economically feasible. Such data sets are required to generate conductivity images using sophisticated inversion software. These capabilities greatly improve the usefulness of the venerable resistivity measurement for problems wherein the resistivity method is traditionally applied. Moreover, the capability of ERT/EIT to generate 'images' of conductivity distribution greatly expands the applicability of the electrical resistivity method to many problems in engineering and hazardous waste site characterization and monitoring, and in geophysical and groundwater exploration as well.

The Inversion of Magnetotelluric Data and the Elimination of Topographic Effects Through Modelling

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Rough topography may cause severe distortion in imaging based on AMT data acquired with the electric field oriented perpendicular to geologic strike, i.e. TM-mode data. Topographic peaks create high-angle conductive distortion while topographic valleys create high-angle resistive features in TM-mode AMT data. Topographic distortions will be carried through to produce high-angle conductive or resistive artifacts in inversion models unless the imaging procedure accounts for the distortion. SCSINV 1-D resistivity-depth images are affected by 2-D topography, while SCS2D 2-D inversions with models including a topographic profile do not.

TE-mode magnetotelluric data, with the electric field oriented parallel to geologic strike, is distorted less by 2-D topography than is TM-mode data. However collecting TE-mode is generally impractical when collecting closely spaced data along survey lines oriented perpendicular to geologic strike. Continuous AMT production is optimized when electric-field dipoles are positioned along survey lines to collect TM-mode data. In contrast, aligning electric field dipoles perpendicular to survey lines to collect TE-mode data is time consuming (and expensive). As a result, most closely sampled AMT data are collected in the scalar TM-mode. Consequently, this paper focuses mostly on the effects of topography upon the interpretation of TM-mode AMT data, although some TE-mode results are included.

Geophysics as an integral part of the Aquifer Storage and Recovery (ASR) process

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Purpose: Develop a rationale for electrical resistivity surveying as a means for mapping ASR (Aquifer Storage and Recovery) bubble morphology during an ASR injection/withdrawal cycle.

Background: Bubble morphology is often assumed to have radial symmetry when the ASR process is modeled. While density variation and its effect on cross-sectional shape is sometimes included, too little attention is given to radial asymmetry due to fracture-dominated permeability, lateral variations in limestone mineralogy, or regional groundwater flow. The net combination of these could influence bubble growth such that the 'real' bubble shape could depart significantly from radial symmetry. We propose that electrical resistivity surveying is a tenable means of mapping bubble morphology/growth characteristics from the ground surface without impacting the ASR process itself.

The effect of electrode contact resistance on electrical field measurements

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A simple equivalent circuit model and field measurements show that dipolar electric field measurements can be changed by up to 50% due to the effects of electrode contact resistance (RC). The equivalent circuit model shows that a high RC enhances the effective wire-to-ground capacitive coupling, leading to a complex dependence of received voltage on frequency, electrode contact resistance, wire length, and wire capacitance. The model shows that measured electric field voltages will fall between a perfectly grounded asymptote (RC -> 0) and an ungrounded asymptote (RC -> infinity). Field tests were made of this model using the controlled source audio-frequency magnetotelluric (CSAMT) technique. By varying the effective RC and the signal frequency, the behavior predicted by the model was confirmed. The tests indicate that electrode contact resistance or ECR effects cannot be ignored in CSAMT data, and that they may influence complex resistivity measurements in certain conditions.

A simple, workable solution to the ECR problem was devised by inserting a high-impedance amplifier in series with the electrodes and by shielding the lead wires, grounding the shield to a common-mode reference pot. Measurements using this configuration show that ECR effects virtually can be eliminated even at high RC values.

Case Histories of Buried Borehole Detection: An Exercise in Flexibility

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It is sometimes difficult to determine in advance exactly which equipment or technique is best suited to a given project. Although the response of a particular target may be predictable, the local background response may be more difficult to assess until field data have actually been gathered. This problem becomes economically significant when the job site is relatively remote, requiring downtime and/or additional airfreight expenses when the actual field results do not match expectations and a change in equipment systems becomes necessary. An example of this is a recent series of surveys in Indiana and Ohio, in which the goal was to ensure that no abandoned, buried oil wells were present within a prescribed radius of proposed injection wells. After local tests, both the physical survey layout and the data processing techniques were varied in order to detect the various possible targets in the different environments.

Advances in Subsurface Modelling Using Seismic Refraction Data

Download Zonge document

Standard geophysical survey practice using seismic refraction techniques has predominantly produced two-dimensional cross sections of the subsurface. The state-of-the-practice for nearly three decades has been to process refraction data with layer reconstruction techniques using the generalized reciprocal method, time-intercept and other similar techniques. Within the past decade, advancements in the computer technology and the development of tomographic modelling algorithms have greatly increased the ability to detect subsurface anomalous features, increase lateral and vertical resolution, and provide better graphical presentation of the data. Recently, 2D finite-element modelling of seismic data has proven successful to image discrete anomalies such as voids.

This paper presents recent developments in a new approach for processing refraction data, the presentation of subsurface data, and the use of these data after geophysical modelling is complete. The approach adapts numerical modelling using the discrete element method and particle flow code (DEMPFC). The procedure is termed the Geostructural Analysis Package (GAP) which, in its initial stages of development has been optimized for geotechnical applications, such as 2D and 3D seismic refraction data processing and presentation on engineering projects. Although GAP has not been primarily created for seismic refraction, this paper will illustrate significant advancement in refraction data processing. Currently, using GAP for seismic applications represents an innovative approach that includes improved data analysis processes and produces more functional result for the end users. For the application illustrated in this paper the end users are typically civil or geotechnical engineers. The value of using this approach for seismic applications is its ability to produce 2D, 2.5D and 3D models to assist engineers or geologists extract additional information from the geophysical data (e.g., material properties), or perform static and dynamic stress analysis. This paper makes the point that mapping the top of bedrock may be the objective of a geophysical survey, but it is not the engineering purpose for the site investigation (e.g., construction of a critical facility, design of a foundation for a structure, etc.). With high-quality calibrated 2D, 2.5D and 3D DEM-PFC models, not geophysical images, engineers are more likely to use the seismic results by incorporating them directly into their engineering analyses.

Results from two case histories are presented showing the benefit of assessing seismic refraction data using the DEM-PFC numerical modelling approach. In the first example, standard 2D refraction data were analyzed and the interpolated results were presented as a 3D model. The second example is a 3D surface tomography reconstruction of four slightly offset 2D refraction shot lines.