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A forum on infrastructure: Unique challenges for infrastructure in the central United States from low-level seismicity
Review of Magnetic Modeling for UXO and Applications to Small Items and Close Distances
The Physical Dipole Model and Polarizability for Magnetostatic Object Parameter Estimation
Portable Magnetic/Frequency Domain Electromagnetic Induction Sensor System Development
Emergence of a new research and development paradigm : The structured contest
Overview of multimethod geophysical system development for enhanced near-surface target detection, discrimination, and characterization
Front Matter
The front matter contains the title page, copyright page, sponsor page, dedication, table of contents, about the editor, foreword, and acknowledgments.
Near-surface geophysics uses the investigational methods of geophysics to study the nature of the very outermost part of the earth’s crust. Man interacts with this part of the earth’s crust: he walks on it; he drills and excavates into it; he constructs structures on and in it; he utilizes its water and mineral resources; and his wastes are stored on and in it and seep into it. The very outermost part of the Earth’s crust is extremely dynamic—in both technical (physical properties) and nontechnical (political, social, legal) terms—which leads to both technical and nontechnical challenges that are much different than the challenges faced by “traditional” applications of geophysics for regional geologic mapping and for oil and gas exploration (see Chapter 2).
Introduction Associating the phrases Special Challenges and Near Surface, as extracted from the title of this chapter, may not entirely describe a unique, one-to-one mapping relationship as the details of this chapter are revealed. Indeed, some of the challenges to be reviewed are ubiquitous in the world of experimental geophysics. However, trying to acquire data that would reveal very shallow geological detail may exacerbate these problems until they can no longer be ignored. It is one thing to examine seawater from the deck of a ship, quite another while swimming in it. While we have not really changed our frame of reference, by being so close to that which we are trying to observe, our “camera” begins to have problems. The action-at-a-distance physics techniques we are trying to employ are barely “at a distance.”
Introduction Geophysical methods can be used to obtain an image of the near surface of the earth. This image will display the spatial variation in the geophysical properties of the subsurface. In general, our interest is not in these geophysical properties, but in the physical, chemical, and biological properties and processes in the earth. To extract this type of information from our geophysical data, we need to transform our geophysical image into an image of these material properties.
Introduction The near-surface is covered with soil in most onshore and offshore locations. Soil characterization by sampling and in-situ testing techniques (e.g., cone penetration and pressure meters) faces unavoidable perturbation effects. On the other hand, low-power geophysical techniques cause no appreciable perturbation and provide an effective alternative for site assessment. In particular, near-surface site characterization using elastic and electromagnetic perturbations yields important information related to the soil mass, including the spatial distribution of materials, small-strain elastic properties and electromagnetic characteristics. In turn, geophysical measurements can be associated with soil parameters relevant to geotechnical engineering analysis and design.
Introduction Throughout this book there are numerous cases where geophysics has been used to help solve practical environmental, geotechnical, and exploration problems. The typical scenario is first to identify the physical property that is diagnostic of the sought geologic structure or buried object, for example, density, seismic velocity, electrical conductivity, or magnetic susceptibility. The appropriate geophysical survey is then designed and field data are acquired and plotted. In some cases the information needed to solve the problem may be obtained directly from these plots, but in most cases more information about the subsurface is required. As an example, consider the magnetic field anomaly map presented in Figure 2. The existence of a buried object, and also approximate horizontal locations, can be inferred directly from that image. The map, however, presents no information about the depth of the object or details regarding its shape. To obtain that information the data need to be inverted to generate a 3D subsurface distribution of the magnetic material.
Magnetic Methods in Near-Surface Geophysics
The Role of Magnetic Methods in Near-Surface Investigations Magnetic methods have a prominent place in near-surface geophysics for a number of reasons. First, the sources of interest often have strong magnetic signatures; sometimes the only measurable geophysical property of the target is its magnetic field. Some examples will be given later in the chapter. Second, magnetic measurements are comparatively simple, rapid, and completely noninvasive. Although the resolution required for near-surface applications generally makes airborne data acquisition impractical, even this is possible in some cases. Finally, in near-surface investigations magnetic data are often easy to interpret; in some cases, visual inspection of essentially raw data is sufficient. For all these reasons, magnetic methods are widely used in almost all areas of near-surface geophysics. In some subfields, such as buried ordnance detection, magnetic methods are particularly important.
Introduction Near-surface seismology derives much of its identity from the physical characteristics of the near-surface environment. Natural materials encountered at shallow depths possess exceptionally diverse mechanical properties as documented by the classification schemes of soil and rock mechanics. Geologic boundaries across which mechanical properties undergo large and rapid changes are commonly present, most notably the water table and the soil-bedrock interface. Porosity occurs in many forms and at a wide variety of scales, and tends toward relatively high values because of low confining pressures. Water, air, biogenic gases, and fluid contaminants occupy the pore space in spatially varying proportions. Near-surface stress increases very rapidly with depth, but the principal stresses may not align with vertical and horizontal directions. At depths where soils and rocks are saturated with groundwater, significant pore water pressure acts on the solid frame. All of these physical characteristics combine with the nature of the seismic source to determine the near-surface seismic wavefield.
Introduction Seismic methods are geophysical techniques that involve the generation and recording of seismic waves for the purpose of mapping the subsurface. Each method is based on the propagation of waves from an artificial source to a set of receivers, followed by an analysis of the recorded wavefield in terms of subsurface properties. Although seismic methods are conceptually not limited to any particular macroscopic scale, the emphasis here is on source-receiver separations that range from a few meters to a few hundred meters, and on depths of investigation that fall approximately within the same range. These linear dimensions define the domain of near-surface seismology, and focus attention on a portion of the uppermost crust that is of great importance to other geologic disciplines, especially geotechnical engineering and hydrogeology. Site characterization and the delineation of aquifers constitute the primary practical applications of near-surface seismology, thereby explaining the many references to engineering and groundwater methods in the applied geophysical literature.
Introduction With only a few minor exceptions, electrical geophysical methods can be divided into two categories: (1) galvanic source methods, directly coupling or injecting electrical current into the ground via electrodes, and (2) inductive source methods, inducing eddy currents into the ground via time-varying magnetic fields using coils not in direct contact with the ground. Ground-penetrating radar (GPR) can be considered an extreme high-frequency, dielectric-properties–sensitive example of the latter. Self-potential (SP) effects, due to electrochemical mechanisms in the ground, and telluric current methods (where only ambient voltages, induced by natural EM sources such as the oscillating magnetosphere of the earth, are measured) can arguably be included in the former, but are both passive methods and are beyond the scope of this chapter.
Introduction Electromagnetic methods have a special place in the arsenal of geophysical tools available for environmental investigations. They owe their status to five factors. First, many environmental investigations are concerned with detecting the location of contaminants in groundwater and identifying pathways for contaminant transport. These types of problems boil down to learning as much as possible about the fluids in the pore spaces. Contaminants can modify both the electrical conductivity and dielectric constant of the pore fluid, and electromagnetic and electrical methods are the only geophysical methods that are directly influenced by the electrical properties of pore fluids. Second, electromagnetic techniques are sensitive to changes in geology such as rock type, porosity, grain size, fractures, and clay content. This sensitivity allows electromagnetic methods to be used for geologic mapping, which is of great value in developing geological and hydrological models needed for environmental studies. Third, there is a wide range of electromagnetic equipment available for purchase or rental. Over the past 30 years companies such as Geonics, Zonge Engineering, GeoInstruments, Geophex, Iris Instruments, Geosystems, and others have developed modern, reliable instrumentation making electromagnetic surveys relatively easy to perform. Hopefully, this new found ease of data collection does not lull the naïve and untrained into thinking that these methods are without pitfalls. Fourth, electromagnetic techniques are noninvasive. While drilling is often necessary to unequivocally confirm an interpretation, drilling can worsen an environmental problem by puncturing an intact, buried container, or by producing a high permeability flow path for further contaminant migration. Finally, compared to other geophysical methods and to drilling, electromagnetic techniques are relatively inexpensive due to their ease of use and low labor requirements.
Introduction Ground-penetrating radar (GPR) is a relatively young geophysical technique. First uses appeared in the 1960s with radio echo sounding of glaciers and ice sheets (Bailey et al., 1964) followed by permafrost analysis (Annan and Davis, 1976). Applications spread with major changes commencing in the 1990s. The history of GPR is intertwined with the diverse applications of the technique. GPR has the most extensive set of applications of any geophysical technique leading to a wide range of spatial scales and concomitant diversity of instrument configurations. A chronological history can be found in Annan (2002). The accompanying references provide further insight into the technology evolution.
Introduction Many geotechnical, environmental, and hydrological investigations require information about soil, sediments, bedrock and groundwater. Boreholes are drilled to investigate these materials in the subsurface, and borehole geophysical measurements are one of the primary methods for determining subsurface properties. Borehole geophysics provides measurements of subsurface properties under in-situ conditions, with no missing samples, and using several different physical measurements. Some of these measurements can be directly linked to noninvasive surface soundings, and a number of new borehole geophysical techniques are under development.
Introduction Carbonate formations generally have broad pore size distributions, from microcrystalline pores to large vugs. Understanding these pore spaces and their geometries is crucial to hydrocarbon reservoir characterization, and because carbonates form aquifers in many regions, an understanding of their pore systems is critical to an understanding of hydrological processes, as well.