Historically, geophysics has been used to characterize deep exploration targets, such as economic mineralization, oil and gas deposits, or new groundwater resources, in frontier environments that are relatively free of human impact. At the same time, civil engineers, archaeologists, soil scientists, and others have applied the traditional geophysical methods with long-trusted but simple interpretation schemes to detect, classify, and describe buried geological or anthropogenic targets in the shallow subsurface. In recent years however, as the amount of Earth’s land area untouched by human impact has decreased and as the importance of responsible stewardship of Earth’s subsurface resources has increased, a significant body of advances has been made in near-surface applied geophysics techniques and interpretation theory that have caused existing textbooks and monographs on the subject to become outdated.

The present book is designed to bring senior undergraduate and graduate students in geophysics and related disciplines up to date in terms of the recent advances in near-surface applied geophysics, while at the same time retaining material that provides a firm theoretical foundation on the traditional basis of the exploration methods. The plan of the book is to explain the new developments in simple physical terms, using intermediate-level mathematics to bring rigor to the discussion. The sections on data analysis and inverse theory enable the student to appreciate the full execution of applied geophysics, from data acquisition to data processing and interpretation. The material is amply illustrated by case histories sampled from the current, peer-reviewed scientific literature. This is a textbook that students will find challenging but should be able to master with diligent effort. The book will also serve as a valuable reference for geoscientists, engineers, and others engaged in academic, government, or industrial pursuits that call for near-surface geophysical investigation.

While seismic-reflection and -refraction techniques are commonly employed to map near-surface layers, they do not have the high vertical resolution (detection of subsurface structures with length scales of 1.0 m or less) that is required for many applications. Ground-penetrating radar (GPR) can be a suitable geophysical tool in these situations. The technique is used to detect changes in subsurface electromagnetic impedance via the propagation and reflection at impedance boundaries of an electromagnetic wave generated by a transmitter deployed at the surface or, less commonly, within a borehole. Typical GPR frequencies are in the 10 MHz to 1 GHz range, much higher than the frequencies used in the electromagnetic (EM) induction method (see Chapter 8). The popularity of GPR as a near-surface geophysical technique lies partially in the similar appearance of radar sections to the seismic sections that are familiar to many geophysicists (Figure 9.1). Both seismic reflection and GPR are imaging techniques based on wave-propagation principles but there are important differences; these will be discussed in this chapter. Good overviews of the theory and practice of GPR appear in Davis and Annan (1989), Knight (2001), Neal (2004), Annan (2009), and Conyers (2011).

Example. Perchlorate transport in karst.

The occurrence of the perchlorate ion ClO4− in groundwater presents a great risk to human health since perchlorate has long been known to inhibit proper functioning of the thyroid. Beneath the Naval Weapons Industrial Reserve Plant (NWIRP) in central Texas, significant concentrations of perchlorate ions derived from the manufacture of rocket propellant have been detected in groundwater and springs. Hughes (2009) has described a wide-area (~ 500 ha) GPR survey in karst terrain with the goal of mapping subsurface structural features that might be indicative of major pathways for subsurface transport of perchlorate ions. The survey was executed by towing a 50 MHz GPR system for ~ 100 line-km on a sled behind an all-terrain vehicle.

The induced-polarization (IP) and and self-potential (SP) methods of geophysical exploration are based on measurements, normally made at the surface of the Earth, of electric potentials that are associated with subsurface charge distributions. In the IP method, the charge distributions are established by an application of external electrical energy. In the SP method, subsurface charge distributions are maintained by persistent, natural electrochemical processes.

Consider the hypothetical situation shown in Figure 5.1 in which electrical charges are distributed unevenly within the subsurface. Charge accumulations are portrayed schematically in the figure as positive and negative “charge centers.” The charges may be volumetrically distributed or they may reside on mineral surfaces and other interfaces. In either case, the regions where charge is concentrated can be viewed as the spatially extended terminals of a kind of natural battery, or geobattery. The sketch shown in the figure greatly simplifies the realistic charge distributions that occur within actual geological formations but it is instructive for the present purpose. Electrical energy supplied from an external source, or energy that naturally arises from a persistent electrochemical process, is required to maintain the “out-of-equilibrium” charge distributions shown in Figure 5.1. Without an energy input, they would rapidly neutralize in the presence of the conductive host medium, and the geobattery would soon discharge.

The purpose of the magnetic geophysical technique is to explore the spatial distribution of magnetized rocks and buried ferrous metal objects, based on magnetic measurements made at or near the surface, and then to make a geological or anthropogenic interpretation in terms of the objectives of the investigation. The magnetic method is perhaps the oldest of geophysical exploration techniques (Nabighian et al., 2005).

The magnetic method has become widely used in near-surface geophysics for several reasons (Hansen et al., 2005): (a) buried targets or geological structures of interest often have readily detectable magnetic signatures owing to the high sensitivity of modern magnetometers; (b) the measurements are fast, reliable, and non-invasive; (c) magnetic data are often straightforward to interpret using qualitative and quantitative techniques, especially when large amounts of high-resolution data are acquired over a wide area such that a continuous plan view of the site and its surroundings can be obtained.

Surface-wave-based methods involving active or passive sources are used in investigations spanning a wide range of scales from ultrasonic non-destructive evaluation of civil infrastructure to global seismic imaging of the Earth’s mantle. Near-surface geophysical applications with active sources, probing to depths ~ 30 m, are experiencing steady growth (Socco et al., 2010). For the most part, the key information is embedded in high-amplitude, low-frequency Rayleigh waves, i.e. the ground roll that is normally regarded as a source of noise in seismic body wave reflection and refraction studies. Typically, surface or interface waves of various types (e.g. Rayleigh, Love, Scholte, Lamb, and Stoneley waves) are guided and highly dispersive. Recognition of these properties drove the development in the 1980s of the spectral analysis of surface wave (SASW) method (Nazarian and Stokoe, 1986) and, later, the multichannel analysis of surface wave (MASW) method (Park et al., 1999). In these and other related techniques, apparent Rayleigh phase velocity versus frequency curves are first constructed, and then inverted to obtain shear-wave depth profiles. The resulting estimates of shear-wave speed in the shallow subsurface can be interpreted in terms of physical properties such as stiffness, liquefaction potential, and moisture content. These properties are of great interest to geotechnical and construction engineers, soil scientists, and others. In addition, the magnitude of ground shaking in response to a nearby earthquake is highly dependent on the subsurface shear-velocity structure.

Rayleigh waves

Consider a mechanical disturbance within an infinite homogeneous elastic medium. Both compressional (P-) and shear (S-) body waves are generated, as described in the previous chapter. Suppose now the elastic medium occupies only the lower halfspace, such that a free surface is present. In this case there exists also a Rayleigh wave (R-wave) solution to the elastic wave equations (Richart et al., 1970). If the medium is heterogeneous, with spatially varying elastic moduli, the R-wave packet is dispersive. The wave packet can be decomposed by Fourier analysis into its individual frequency components. Each frequency component of the wave packet travels at its own characteristic phase velocity. The shape of the phase velocity versus frequency curve is sometimes called the dispersion characteristic. Note that an R-wave traveling in a homogeneous elastic medium is not dispersive.

This chapter provides a general overview of some elementary concepts in geophysical data analysis such as information, sampling, aliasing, convolution, filtering, Fourier transforms, and wavelet analysis. Good introductions to many of these topics may be found in Kanasewich (1981) and Gubbins (2004). The material presented in this chapter is aimed to help the near-surface geophysicist to process and interpret data acquired in the field using the techniques that are discussed in the subsequent chapters of this book.

Information

Geophysicists gather data from which they attempt to extract information about the Earth, in order to better understand its subsurface structure and the wide variety of geological processes that shapes its evolution. The assumption is that geophysical datasets contain information about subsurface geology. It is useful to establish what is meant by the term information. There are many ways to define information but, at the most fundamental level, I regard it as a quantum, or bit, of new knowledge. Special emphasis should be placed on the word new in the foregoing definition, in order to reinforce the concept that previously existing knowledge is not information.

This chapter highlights a few of the emerging techniques of near-surface applied geophysics. The discussion is designed to provide the reader with a sense of some of the latest developments in this rapidly growing discipline. The emergent techniques studied here include surface nuclear magnetic resonance, time-lapse microgravity, induced seismicity studies, landmine discrimination, GPR interferometry, and the seismoelectric method. There are many other advances being made, or that will be made in the near future, beyond those described in this chapter; the interested reader is advised to keep watch on the topical journals and conferences.

Surface nuclear magnetic resonance

The surface nuclear magnetic resonance (sNMR) technique is a relatively new geophysical method that can directly sense spatial variations in subsurface water content to depths of ~ 150 m. The sNMR technique holds promise to open new and exciting avenues in groundwater resource investigations. The method is based on the interaction of an applied magnetic field with the magnetic moments of the hydrogen nuclei, or protons, in groundwater. The sNMR concept was first described in a patent by Varian (1962), followed by pioneering field work of Russian geoscientists during the 1970s and 1980s.

The seismic-reflection and -refraction methods in near-surface geophysical investigations are based on the introduction of mechanical energy into the subsurface using an active source and the recording, typically using surface geophones, of the resulting mechanical response. Passive-source seismic methods also provide important information; these will be described in Chapter 7. The propagation of mechanical energy into the subsurface consists, to a large part, of elastic waves. The essential property of an elastic body is that it returns instantaneously to its original pre-deformed state with the removal of a mechanical force that changed its size and/or shape. A delayed return to the original state is termed viscoelasticity. Any permanent deformation, such as ductile deformation or brittle failure, is a measure of the inelasticity of the body. Significant permanent deformation of the ground surface can occur in the vicinity of large seismic disturbances such as earthquakes (e.g. Lee and Shih, 2011) but inelasticity can be safely neglected in most near-surface active-source or passive-source studies. An important characteristic of elasticity is the relationship between the strain, or deformation, of a body and the stress, or mechanical force, that produces the deformation of the body. An excellent review of the elementary physics of wave motion is found in French (1971).

Introduction

There are several possible types of elastic wave motion following the introduction of a seismic disturbance. The particle motion associated with compressional, or P-waves, is aligned with the direction of wave propagation (Figure 6.1a). The particle motion associated with shear, or S-waves, is aligned in a direction perpendicular to the direction of wave propagation (Figure 6.1b). Both vertically polarized (SV, as shown in the figure) and horizontally polarized (SH) motions are possible. The P- and S-waves are known as body waves since they are transmitted through the interior of the Earth. As shown by the shaded cells in the figure, P-waves are associated with a change in size and aspect ratio of an elementary material volume while S-waves are associated with a change in shape. With surface Rayleigh, or R-waves, discussed more fully in the next chapter, the particle motion near the surface is retrograde elliptical (Figure 6.1c, top) and only those particles in the region close to the surface of the Earth, at depths comparable to the elastic wavelength, are set into motion. A second type of surface wave motion (Figure 6.1c, bottom) is characterized by horizontal particle motion that oscillates transverse to the direction of wave propagation. Such waves are termed Love waves.

The electromagnetic (EM) induction method, traditionally used for mining, groundwater, and geothermal exploration and geological mapping (Grant and West, 1965; Nabighian, 1988; 1991), is growing in popularity for near-surface geophysical applications (McNeill, 1990; Nobes, 1996; Tezkan, 1999; Pellerin, 2002; Fitterman and Labson, 2005; Everett, 2012). The controlled-source variant of the method utilizes low-frequency (~ 1–100 kHz) time variations in electromagnetic fields that originate at or near the surface and diffuse into the subsurface. The ground-penetrating radar (GPR) technique is distinct from the EM induction method in the sense that the former utilizes a higher frequency range > 1 MHz for which wave propagation, rather than diffusion, is the dominant energy transport mechanism. The diffusive regime is marked by the requirement σ >> ωε where σ [S/m] is electrical conductivity, ω [rad/s] is angular frequency, and ε [F/m] is dielectric permittivity. The latter plays no physical role in the EM induction method. Instead, EM induction measurements respond almost entirely to the bulk subsurface electrical conductivity and, in particular, the spatial distribution of highly conductive zones (Everett and Meju, 2005). Instrumentation is readily available, easy to use, and reliable. Electric fields are sensed by pairs of grounded electrodes, i.e.voltmeters, while time-variations of magnetic fields are most commonly sensed using induction coils.

Introduction

The electrical conductivity structure of the subsurface, as determined from EM induction measurements, can be interpreted in the context of a wide variety of potential targets and application areas, see Table 8.1. Fundamentally, EM methods respond with good sensitivity to fluid type, clay content, and porosity.