Beyond the Wellbore Profile: Understanding Subsurface Deposits with Tomographic Data

One of the most common methods for obtaining estimates of 3-D aquifer structures is through interpolation of (1-D) wellbore profiles of hydrologic, visual, or geophysical properties. The ease of data collection, the ability to use relatively simple data analysis methods, and the intuitive nature by which these interpretations are mapped into 3-D aquifer volumes (interpolation) make wellbore profiling methods very attractive. Tomographic methods for aquifer characterization represent an entirely different approach to the aquifer characterization problem. In tomographic methods, an aquifer is stimulated at a particular location (the source, which can be hydrologic, chemical, or geophysical in nature), and response data is recorded at a number of different observation locations (the receivers). Because tomographic data is sensitive to aquifer parameters over a larger support volume, it can be more valuable for obtaining information about connectivity of geologic features, which is important for contaminant transport and other applications. However, key investments must be made to collect useful tomographic data. A large set of instrumentation may be needed to collect data at numerous receiver locations. And likewise, the computational demand required to analyze tomographic data for aquifer imaging is generally much higher than for profiling analyses.
            In this presentation, I review several key advancements that are helping to move tomographic techniques toward more widespread application. In particular, I will review instrumentation, test design, and computational advancements that are helping to improve the value proposition of hydraulic tomography – a tomographic method that uses data from pumping interference tests to infer 3-D heterogeneity throughout an aquifer. I will also demonstrate, through synthetic applications, how tomographic methods can extract detailed heterogeneity information beyond what could be obtained with simple wellbore profile interpolation. As hydrogeology moves into the 21st century, tomographic methods represent a key way in which we as a community can take better advantage of the new opportunities presented by rapidly increasing sensing and computational capabilities. Finally, to encouraging further advancement of the field, I will discuss the important role of industry collaborations and support for method development and standardization.