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Executive Summary Knowledge of the nature of the subsurface is critical for many environmental purposes and engineering applications. Projects requiring subsurface character- ization are estimated to cost several trillion dollars over the next few decades. There is a need to "see into the earth," to determine physical, chemical, and biological properties and to detect, monitor, and predict natural and induced processes. Direct or invasive methods of characterization (such as drilling and excava- tion) often can be expensive, cause disruption to human activities, and in some cases, create unnecessary damage to the environment. Drilling provides, in es- sence, a one-dimensional sample; multiple drillholes allow interpolation between such points but significant uncertainties can remain. The primary questions addressed by this study are: (1) How effectively can the shallow subsurface be imaged and characterized noninvasively? (2) Can the use of noninvasive methods increase the confidence of our characterization effort? A large suite of noninvasive methods has been developed and refined over the past few decades. Noninvasive methods for characterization of subsurface properties and processes are indirect. Interpretations are based on measured re- sponse of the subsurface to artificial or natural stimuli. Passive investigations use naturally occurring fields (the earth' s gravity, magnetic, electric, thermal, radio- metric, stress, solar irradiation, and hydraulic fields). For example, perturbations in the earth's gravity field are used to infer changes in the material density. Active investigations use a source of energy (e.g., seismic energy, radar pulses, electrical inputs) that creates a known field, and observations are made of the perturbations in that field or in the response of the earth. For example, seismic investigations use vibratory or explosive sources to propagate elastic waves, and 1
2 SEEING INTO THE EARTH travel times, wavelet changes, and scattering are measured to describe the hetero- geneity of the shallow subsurface as well as the interior of the earth. There is great potential for these methods to define subsurface details with a level of accuracy, precision, economy, and safety that can approach direct sam- pling but with a much greater areal coverage. (At all sites one needs to be cogni- zant of the various types of noise or other disturbances that might diminish the utility of one or more specific techniques.) Realizing the potential for noninvasive characterization will require concerted and cooperative interdisciplinary efforts by earth scientists, geotechnical engineers, government agencies, and the user community. Two broad areas of effort are discussed. First, additional research and development would improve and extend the capabilities of many noninvasive meth- ods. Second, existing tools and methods are quite adequate for many characteriza- tion activities but are not being widely used in practice for a number of reasons- a major focus of this report is on improving the use of existing methods. RESEARCH AND DEVELOPMENT During the past two decades, advances in computing and microelectronics stimulated the production of an impressive array of tools and techniques for noninvasive characterization of the shallow subsurface. For the most part, these advances made existing tools and techniques faster, cheaper, or more effective. There have been relatively few fundamental innovations with regard to the phe- nomena being observed or the sensing devices that convert these phenomena into electrical signals. Additional research and development (R&D) is needed to en- hance and extend current capabilities and to develop new measurement tech- niques (see Box 1~. Noninvasive site characterization would probably be used more frequently and efficiently if much of the data acquisition, data processing, and decision making could be automated. Automation, which will not replace skilled practitio- ners, could significantly increase the knowledge base that practitioners can use to accomplish their jobs. By producing a better result, more rapidly and at lower cost, robotics and decision support systems could be the key to more and more effective use of noninvasive site characterization methods. Automation of site characterization allows measurements and preliminary interpretations to be made in real time. Characterization problems are complex and multifaceted. Noninvasive meth- ods, ranging from electromagnetic and seismic techniques to remote sensing by aircraft and satellites to on-site soil-gas surveys, must be selectively used to provide complementary information that enhances our ability to resolve subsur- face features. Basic research on physical, chemical, and biological properties is needed to establish fundamental relationships, including coupled relationships, that improve capabilities for rigorous interpretation. A significant effort should be directed to develop scientific visualization technology that is aimed at substan
EXECUTIVE SUMMARY 3 tially enhancing three-dimensional interpretations based on multiple data sets. In addition, visualization technologies will help make the interpretations easier for decision makers to understand. The spatial and temporal resolution demanded in a specific characterization study is a major factor in the choice of method, and resolution normally is a function of the scale of the survey. An understanding of resolution requirements and capabilities is essential to informed choice and application of specific tech- niques and will provide important guidance in establishing research priorities. Ongoing basic research is essential if we are to develop high-resolution tech- niques for application in a variety of geological conditions. Testing of data acqui- sition and analysis procedures to assess and improve the capabilities of existing technologies is needed. Integration of data from other invasive and noninvasive methods can be used to "verify" new capabilities, but one has to ensure that validations do not involve circular reasoning. Historically, site characterization has focused on mapping the geometry of
4 SEEING INTO THE EARTH the subsurface (e.g., location of anomalies, shape of boundaries). However, physi- cal, chemical, and biological properties and processes may be as important as geometry and require greater emphasis in both research and practice. Fundamen- tal studies should be expanded and developed to establish theoretical and phe- nomenological relationships among the responses measured using noninvasive methods and the properties and processes of interest in environmental and engi- neering problems of the shallow subsurface. These studies could include con- trolled standard test sites and variable-scale testing that would involve both labo- ratory testing of cores and full-scale multitechnique examinations of relevant field sites. In many cases especially those involving groundwater, waste management, and hazards subtle changes from a preestablished baseline are more important than current conditions. Fortunately, properly designed surveys often can moni- tor changes with high resolution. Rigorous computer models based on an understanding of physical, chemical, and biological processes are underutilized in the understanding and interpretation of site investigation measurements. Modeling is necessary to allow firm linkage of the parameters of interest to the properties and processes occurring in the earth. Models could allow optimization of survey design, resolution of uncertainties and limitations associated with data acquisition, and validation of interpretations. There are many existing computer modeling programs that are potentially useful, but they must be catalogued, documented, and made both user friendly and easily accessible to fulfill their potential. Much of this report focuses on the improvement, or improved use, of exist- ing technologies. These technologies primarily measure physical properties and processes; a few measure selected chemical properties and processes; and cur- rently, few if any are routinely applicable to subsurface biological properties and processes. There exists tremendous potential for the development of new meth- ods that would allow the acquisition of more information about the chemical and biological state of the subsurface. To date, there has been little communication among geochemists, geophysicists, and geobiologists regarding the possible ex- tension of existing measurement techniques to determine a different type of prop- erty (e.g., geochemical measurements to determine geobiological processes). New development has to focus on measuring new chemical and biological properties and processes as well as better processing, modeling, visualization, and concur- rent utilization of the data from all measurements. Controlled test sites should be established for long-term research and used to validate the measurement-data processing-modeling-interpretation systems and to facilitate regulatory approval. PRACTICE Many current needs for near-surface characterization can be met with exist- ing tools and techniques, but these tools and techniques are not widely used.
EXECUTIVE SUMMARY s Much of the site characterization conducted today uses techniques that are more than 20 years old. Widespread adoption of effective tools and techniques that currently exist offers the single greatest opportunity for dramatic, short-term improvement in site characterization (see Box 2~. Many clients and practitioners often exclude noninvasive tools in their arsenal of methods for subsurface characterization. For the demand for noninvasive tech- nologies to increase, the techniques must be demonstrated either as being cost- effective or as providing information that is unavailable any other way. However, many potential clients today perceive characterization tools as increasing costs without providing a commensurate additional value. Their primary objective com- monly is to satisfy externally imposed requirements (e.g., building codes, environ- mental regulations) at minimum cost. Many clients and practitioners rely on "tried- and-true" characterization methods rather than innovative noninvasive methods. To overcome this lack of incentive, case studies should be publicized that analyze the possible cost-effective use of noninvasive site characterization. The gap between the state of knowledge and the state of practice in noninva- sive methods can be addressed by improved communication with and education of practitioners (e.g., contractors who conduct the measurements in the field) regarding advances in techniques and methods. In addition, communication and education on noninvasive characterization methods should be extended to clients and regulators (who write specifications) and the general public. These efforts should include expanded university curricula and training, continuing education opportunities for practitioners and users, and general public education.
