Introduction

Seismic investigations utilize the fact that elastic waves (also called seismic waves) travel with different velocities in different rocks. By generating seismic waves at a point and observing the times of arrival of these waves at a number of other points on the surface of the earth, it is possible to determine the velocity distribution and locate subsurface interfaces where the waves are reflected or refracted.

Traditionally, seismic methods are classified into two major divisions, depending on the energy source of the seismic waves: (1) Earthquake seismology, and (2) Explosion seismology. The main contributions of earthquake seismology are information on the physical properties and structure of the earth's interior. In a geotectonics and civil engineering context, important applications include studies of earthquake magnitude and frequency relationship, evaluation of seismic hazards, and prediction of earthquakes and mitigation of damages. These topics will not be discussed because earthquake seismology is not included in the scope of the book.

Methods of explosion seismology (the reflection and refraction survey methods) are widely applied to studies of crustal structure, geological correlation of layered sequences and mapping of structures in the uppermost crust, and to hydrocarbon exploration. In the same way, but on a smaller scale, these methods are applied to environmental and geotechnical problems such as location of water table, waterbearing fracture zones, cavities and sinkholes, delineation of faults and fractures, and site investigations of foundation conditions including determination of depth to bedrock.

Introduction

Since the discovery of the law of gravitation (Isaac Newton, 1685), the mutual attraction between all masses has been recognized as a universal phenomenon. The phenomenon accounts for the familiar fact that a body when released near the earth will fall with increasing velocity. The rate of increase of velocity is called the gravitational acceleration, or simply gravity, g, which would have a single constant value on the earth's surface if the earth were a perfect sphere of uniform concentric shell structure. In fact, our earth is non-uniform, non-spherical, and rotating, and all these facts contribute to variations in gravity over its surface.

The measurements and analyses of the variation in gravity over the earth's surface have become powerful techniques in the investigation of subsurface structures at various depths. Of prime interest in environmental and engineering studies are the variations in gravity which reflect lateral density contrasts associated with targets at shallow depths. In many cases the density contrasts occur at boundaries between different geological formations, although man-made boundaries such as tunnels and mines also represent contrasts. Gravity variations caused by density inhomogeneities are relatively small (often <10-6 of the earth's gravity), and their accurate measurement is possible only with the aid of extremely sensitive instruments.

As a geophysical technique, the gravity-surveying method has a great deal in common with the magnetic method. Both gravity and magnetic fields are potential fields, and both require fundamentally similar interpretation techniques.

Geophysics and the earth's environment

Geophysics is the study of the planet earth using methods of physics. However, the term is often used in a restricted sense to denote the physics applied to the study of the ‘solid earth’ (excluding the hydrosphere and atmosphere). Solid earth geophysics is traditionally divided into two main fields of study: (1) global geophysics, and (2) exploration geophysics. The first involves studies of large-scale problems relating to the earth's gross structure and dynamic behavior, whereas the second deals specifically with applications of geophysical techniques to problems of oil/gas, water, and mineral exploration.

Geophysical exploration has rapidly expanded over the past 50 years and has helped to cope with increasing needs for raw materials and energy resources required by modern civilization. However, in its wake this has created new geophysical tasks relating to environmental protection. The current environmental problems include, among others, location of buried hazardous waste material, protection of soil and groundwater from contamination, and planning of new, safe and secure disposal sites for dumping of nuclear, industrial, and domestic waste.

In addition, as mining, oil and gas production, nuclear power installations, earthworks and heavy constructions continue to expand, various engineering/geotechnical problems are also demanding increasing attention from geoscientists and engineers. Integrity and safety of critical man-made structures such as nuclear waste repositories, dams, subways, etc., are of great importance both from engineering and environmental considerations.

Introduction

Ground-probing radar (GPR), also variously known as ground-penetrating radar, ground radar or georadar, is a high-resolution technique of imaging shallow soil and ground structures using electromagnetic (EM) waves in the frequency band of 10–1000 MHz. The advantage of using EM waves is that signals of relatively short wavelength can be generated and radiated into the ground to detect anomalous variations in the dielectric properties of the geological material. In many respects, GPR methodology is similar to that of shallow seismic reflection surveying. Both methods use reflection of energy from underground features but they differ largely in their site-specific applicability. Radar will not penetrate into materials of high electrical conductivity, such as damp clays, which are excellent targets for the seismic reflection technique. On the other hand, radar penetrates dry sand and gravel that will not easily transmit high-frequency seismic waves. An important distinguishing feature of GPR is that the method is easy to use and is neither destructive nor invasive; this makes it suitable for use also in urban settings and archaeological environments.

A ‘young’ geophysical exploration method, GPR is only about 25 years old but has proven to be useful for a wide variety of environmental and engineering problems where the target depths are relatively shallow. The method is most effective when working with low-attenuation media (characterized by low electrical conductivity) such as ice, sand, crude oil, bedrock, fresh water, etc., and less effective or even ineffective when working with high-attenuation media such as wet clay/silt, salt water, etc.

Introduction

The study of the earth's magnetism is the oldest branch of geophysics. It is generally believed that the Chinese were the first to make use of the north-seeking property of ‘lodestone’ (a magnetite-rich rock piece). However, the idea that the earth itself acts as a magnet was realized much later. It was in the year 1600, when William Gilbert published his book De Magnete, that the concept of the earth's magnetic field and its directional behavior was put on a scientific footing.

The first systematic studies of local anomalies in the direction of the earth's field were made in Sweden for iron-ore prospecting probably as early as 1640 and regularly by the end of that century. However, it was not until the late 1870s that special instruments were developed by Thalén and Tiberg for routine use in prospecting surveys. Until about the 1940s the magnetic field instruments were tedious to operate and most of the field operations were restricted to small-scale land surveys. The development of the fluxgate magnetometer (during World War II) and the proton magnetometer in the mid 1950s brought about a revolutionary change in the speed of operations that made large-scale surveys possible by using airborne and ship-towed magnetometers.

Introduction

The self-potential (SP) method is based upon measuring the spontaneous or natural potentials developed in the earth by electrochemical actions between minerals and subsurface fluids or by electrokinetic processes involving the flow of ionic fluids. It differs from other geoelectric methods (Resistivity and Induced Polarization (IP) methods discussed in Chapter 6) which require electric currents to be artificially introduced in the ground. The SP method is one of the simplest and oldest among all geophysical techniques used to locate sulfide ore-bodies.

In recent years the SP method has found increasing use in geothermal, environmental, and engineering applications to help locate and delineate sources associated with the movement of thermal fluids and groundwater. Specific applications include mapping of seepage flow associated with dams, dikes, and reservoir floors, and delineation of flow patterns in the vicinity of water wells, faults, landslides, and sinkholes. As the method offers relatively rapid field data acquisition, it is often cost effective for initial investigation of an area prior to more intensive studies using other geophysical methods.

The following discussion is an overview of the principles and practice of the method and includes some examples of environmental and engineering applications.

Origin of self-potentials

Spontaneous potentials in the subsurface are caused by a number of processes which are not well understood at this time. However, the principal types of source mechanisms appear to be as follows.

Electrofiltration potential

The flow of fluid through a capillary or porous medium may generate an electric potential (called the electrofiltration or streaming potential) along the flow path.

Introduction

It has been known for many decades that temperature, as measured in mines and boreholes, increases with depth. This suggests that the earth's interior is warmer and, therefore, that heat must be flowing upward in the earth. This outflow of heat is almost imperceptible on the earth's surface except at a few localities where heat is sometimes dramatically transferred from the earth's interior to the surface through volcanoes and hot springs. The main source of heat energy within the earth is probably the radioactive decay of long-lived isotopes. A small portion may be a residual effect from the processes that formed the earth.

Worldwide studies of heat flow have provided information on the broad characteristics of thermal conditions beneath the major geological features, including continental rifts, oceanic ridges, subduction margins, and intraplate zones of anomalous thermal activity. Geothermal studies on a regional scale have become all the more important since the energy crisis of the early 1970s when it became imperative to consider geothermal energy as one of several alternative energy sources to displace the use of oil. The broader aspects of geothermal studies including regional heat flow and temperature distribution with depth, exploration for geothermal resources, and heat extraction from thermal reservoirs are discussed in Rybach and Muffler (1981) and Jessop (1990).

On a local scale, thermal measurements (in particular temperature anomalies) have been used to infer the positions of structures such as shallow salt domes, anticlines, faults, fissures, etc.

Introduction

Geophysical borehole logging involves measuring the physical properties of surrounding rocks with a sensor located in a borehole. The record of measurements as a function of depth is called a borehole log (or well log). Many of the methods of geophysical surveying (described in previous chapters), with some modifications, can be adapted for use in borehole logging. Downhole geophysical measurements greatly increase the value of the hole to the interpreter by their ability to provide more direct information about formation thickness and lithology, dip of strata, porosity, formation fluid content, permeability, and temperature. In addition, they provide a means of correlating geological formations from one borehole to another. Geophysical logging techniques have been widely used in the investigation of wells drilled for hydrocarbon exploration, as they provide important in-situ properties of possible reservoir rocks.

This chapter focusses on the application of geophysical logging techniques that are of special relevance to environmental investigations. These are related mainly to the current and future stability of the site and the possible migration of contaminants in the groundwater. In particular, environmental impact studies at proposed landfill sites and potential nuclear waste repositories have concentrated much attention on detecting fractures as possible conduits for rapid transport of toxic chemicals. Conventional logging techniques (such as those used in exploratory wells for oil/gas and water) can be applied to define the hydrogeological regime at hazardous waste sites, but special interpretation procedures must be applied in those areas where the logging zone of interest is above the water table, or where the rock has an extremely low permeability.

Over several years of engagement in geophysics teaching and research at the University of Copenhagen, I often felt a need for a comprehensive text on applications of geophysical methods to a wide variety of environmental and engineering problems that range from protection of soil and groundwater from contamination, safe disposal of chemical and nuclear wastes, to geotechnical site testing for underground vaults, subways, dams, reservoirs, and power plants.

Hitherto available literature on this subject is almost exclusively in the form of case histories published in the proceedings of various symposia (such as SAGEEP proceedings published by the Environmental and Engineering Geophysical Society) or special issues of some geophysical journals, and a few special volumes published by the Society of Exploration Geophysicists. To my knowledge, there is not yet any textbook that adequately covers the area of geophysical imaging of the shallow subsurface from the standpoint of environmental and geotechnical problems. The present work is an attempt to fill in this gap.

The structure of the book is summarized in the following. An introductory chapter outlines the basic principles of geophysical methods and their main areas of application in environmental and engineering contexts. The succeeding ten chapters describe all of the principal methods of geophysical surveying, each chapter containing the necessary basic theory, field procedures, interpretation techniques, and examples drawn from case histories. A final chapter is devoted to inversion theory and tomography to help understand the basis of geophysical imaging.

Introduction

In the discussion of various methods in the previous chapters several examples of interpretation of geophysical field data have been given where ‘inverse’ modeling techniques were employed. Whilst the forward theory for computing predicted responses of various types of models has been discussed previously in varying detail, the inverse theory received only a cursory treatment. Also, with regard to tomographic investigations, very little has been said about the underlying theory. In this chapter we fill in these gaps.

There is a wide variety of inverse techniques for application to geophysical problems. The level of application of inverse theory may range from the simple straightline fitting of seismic refraction data to more sophisticated acoustic or electromagnetic tomography or multidimensional resistivity sounding interpretation. The field is too broad to be treated in a comprehensive way in a single chapter. Nonetheless, an attempt is made to provide the basic elements of some commonly used inversion techniques in the following sections. The later part of the chapter is devoted to tomographic imaging, which is treated as a special problem of the inverse theory.

The inverse problem and its formulation

In contrast to the ‘forward modeling’ approach, where the interpretation of field data involves comparison with theoretical responses of mathematical models with assumed sets of parameters, the basic approach used in ‘inverse modeling’ may be formulated as follows: Given some data on the values of some measured quantities (e.g., potential field or travel-time data), we use a theoretical relationship to derive the values of the set of parameters of a model that reproduces the given field observations.

Introduction

Electrical measurements of various types are made at the surface of the earth to investigate subsurface conditions in an area. In the most commonly used method, an electric current is driven through the ground and the resulting potential differences are measured at the surface. Anomalous conditions or inhomogeneities within the ground, such as electrically better or poorer conducting layers, are inferred from the fact that they deflect the current and distort the normal potentials. This is the underlying principle of measuring subsurface variation in electrical resistivity (reciprocal of electrical conductivity) within the earth. The technique of resistivity surveying was developed by Conrad Schlumberger, who conducted the first experiments (1912) in the fields of Normandy.

Electric potentials in the ground can, however, be caused by phenomena other than the simple ohmic conduction of the applied current. For example, electric charges sometimes accumulate on the interfaces between certain minerals as a result of the flow of electric current from an external source. The method of ‘induced polarization’ (IP) is based on this phenomenon in the search for disseminated ores, clay minerals, and groundwater. Moreover, electric potentials can be developed in the ground by electrochemical actions between minerals and the solutions with which they are in contact, or by electrokinetic processes involving the flow of fluids (discussed in Chapter 5). In addition, slowly varying potentials are caused by natural (‘telluric’) currents flowing inside the earth.