Applied geophysics, uses physical methods (such as seismic, gravitational, magnetic, electrical and electromagnetic) at the surface of the Earth to measure the physical properties of the subsurface, along with the anomalies in those properties. It has been extensively used, historically, for oil and gas exploration, closely followed by mineral exploration. The depths explored are typically in hundreds and thousands of meters. Engineering geophysics is often used in construction sites to solve a variety of geological, geotechnical, or quality control problems and helps fill gaps in geotechnical data. The basic task of engineering geophysics is to apply physics theories and methods to determine subsurface conditions for building foundations and test quality of man-made structures (bridges, roads, dams, etc). Dam geophysics has been a late comer to the applied geophysics world although there is nothing fundamentally different. Most of the big dams are very old and regular monitoring and maintenance of these dams is of utmost importance for continuing benefits. Unlike soil investigations, critical nature of dams, does not permit traditional invasive inspections by means of drilling, and such inspections are best avoided unless extremely important, and are done only when problem is too grave. Although long recognized that dams need periodic inspection and monitoring, it has only been recently recognized that geophysical surveys can supplement the results of standard inspection and monitoring techniques. Geophysical surveys have been performed on a number of dams around the world, including India, and have yielded extremely useful insight into dam conditions. The choice of right tool, timing of such investigation and deliverables expected are, however, not very well defined at the moment. Sub-surface imaging by means of geophysical survey is a powerful tool for subsurface mapping which historically has been under-utilized world-over. Continuing improvements in survey equipment performance and automation have made large area surveys with a high data sample density possible. Advances in processing and imaging software have made it possible to detect, display, and interpret small geological features with great accuracy. This has made dam geophysics extremely attractive, especially due to the fact that dams do require periodic condition monitoring using non-destructive tools. In the event of a dam failure, the economic loss as well as the potential hazard to life and property could be enormous. Typical dam safety surveillance consists of visual inspections supported by limited instrumentation. However, the problems in dams can become quite advanced before the problem is detected via these means. Recently, interest has grown regarding the use of non-intrusive geophysical techniques to facilitate early detection of anomalous seepage, piping, internal erosion and other degradation issues. Geophysical methods are sensitive to contrast in the physical properties in the subsurface. Different methods respond to different physical properties, like material strength, material conductivity (linked to water saturation), fluid movement (seepage), change in density etc. The application of geophysical methods to dams enables detection of problems in early stages and hence can become part of dam safety surveillance program. Geophysical techniques, by virtue of their non-invasive and non-destructive nature, offer an excellent solution for investigation or regular monitoring of dams, and detection of anomalous conditions which might snowball into major problems if left untreated. Various geophysical methods are available to investigate the problems of earthen, masonry, concrete or composite dams: Leak path detection Internal Erosion Identification of zone of water accumulation Cavity/sinkhole Concrete degradation No single geophysical technique can uniquely solve the problem due to a large overlapping of physical properties in various subsurface materials. To date, the use of geophysical methods to investigate dams has produced mixed results, partly because the application of these methods is not well-understood and partly because false positives cannot be tolerated. That is the reason why it becomes important to use a combination of geophysical methods to uniquely resolve the problem.
Choosing the right tool/technique to address to a specific problem is critical for success of a geophysical program. A detailed geophysical investigation plan must be worked out in consultation with the client, to address to critical issues. The right combination of various available tools should be chosen to resolve the problem in unique manner. Surveys which are both successful and costeffective must satisfy a number of basic requirements. They must be implemented using appropriate and properly configured survey equipment. The data sampling strategy and density must be matched to the spatial resolution and statistical requirements of the survey. Monitoring the quality of data while in the field is mandatory, and post survey data processing must be both appropriate and mathematically sound. The chosen geophysical contractor should have experienced personnel and advanced instruments to carry out high resolution geophysical surveys for geotechnical investigations.
The survey methodology and tools to be used should be decided in consultation with the client and depending on site conditions and objectives of survey. A technical report describing the work, including high resolution maps with detailed interpretation, should be presented to the client upon completion of analysis. Present paper attempts to define dam geophysics, establishes relevance and importance of geophysical methods for monitoring and health checks of dams, and also suggests methodology to select the right tools and execute dam geophysics projects.
