High-resolution seismic reflection to image hydrogeologic sequences

Introduction High-resolution seismic reflection has been successfully used to delineate preferential pathways within groundwater systems that represent corridors for contaminant movement in a wide variety of challenging hydrologic settings, many in settings where other methods and/or monitor wells have not been enlightening. For more than four decades, seismic reflection has found utility addressing near-surface groundwater problems where lateral variability of confining layers have inhibited even the most sophisticated flow models developed from monitor wells (Shepers, 1975). Optimizing acquisition and processing parameters while limiting interprettations to only what can be validated on shot records and consistent with wave propagation and reflection theory is essential for making meaningful and reliable contributions to a groundwater flow model and associated monitoring and remediation programs (Steeples and Miller, 1990). Years of application of the method in a wide range of settings and with a diverse set of imaging objectives has resulted in an excellent collection of case studies that provides guidance for current and future applications and developments as well as evaluations of method feasibility. The seismic reflection method has been used to establish lateral continuity in confining units with thick dry sandy overburdens as well as fine-grained unconsolidated and saturated near-surface settings. Mapping bedrock is an important application where percolation rates in the vadose zone are high and bedrock units are impermeable. Seismic reflection is a viable tool for studying sitewide variability in unconsolidated alluvial sediments where complex vertical migration paths can allow contaminants to easily move between local “confining” layers, leaving zones directly beneath a source contaminant free, while deeper layers, seemingly protected by several aquicludes, are rich in contaminant. The complexity of many depositional settings results in rapid vertical changes in material properties and therefore a need for high-resolution imaging that does not require a priori information or assumptions about the sequential nature of the vertical property changes. Even with seismic reflection’s many positive and sometimes amazing attributes and capabilities, it is imperative that an awareness of the method’s limitations and true potential in real-world settings be maintained. Near-surface seismic methods do not lend themselves to distinguishing different types of liquids within a groundwater system. For example, distinguishing DNAPLs or LNAPLs from within a saturated interval is beyond the resolution of the seismic tool in real-world settings. However, interrogation of the subsurface in search of lithologies or structures that might represent traps for contaminants has proven very effective. In fact, based on the properties of liquids in the subsurface, it is many times possible to infer traps and likely areas of high concentrations based on mapped reflector structures in conjunction with well control.