ULTRASHALLOW SEISMIC REFLECTION MONITORING OF SEASONAL FLUCTUATIONS IN THE WATER TABLE

Detailed characterization of the shallow subsurface is important in environmental, groundwater, and geotechnical engineering applications where the location and temporal behavior of the water table is important, such as during flooding, construction, or at remediation sites. Drilling or trenching to assist in characterization, however, is at times imprudent or prohibitively expensive. In such cases, characterizing the upper few meters of the earth cost-effectively and noninvasively becomes important. Ultrashallow seismic reflection (USR) imaging offers an alternative tool in instances where ground-penetrating radar or other geophysical techniques are not sufficient for detailed site characterization at shallow depths (i.e., within 3 m of the surface).

Ultrashallow seismic-reflection data were collected at a test site in Great Bend, Kansas. The purpose of the experiment was to image seasonal submeter-scale fluctuations in the water table over a period of one year to identify the factors important in monitoring the water table when using seismic-reflection techniques. The study indicates that 1) detailed velocity information must be available for the entire volume above the water table, and 2) depth (not time) sections must be used when interpreting water-table levels. Using detailed velocity information as a control when depth-converting the seismic profiles yielded correct positioning of the water table within ±12 cm at the test site. Clearly, seismic time sections should not be used to examine the water table at depths shallower than 3 m at sites where near-surface velocity structure is complicated or time varying. The USR time-sections described here did not yield usable information about the depth of the water table, even in a relative sense. Also, a time-to-depth conversion using only the direct-wave velocity and the water table reflection information did not provide accurate sections. Correct monitoring of the water table was only possible when additional velocity information about the velocity structure of the entire volume above the water table using multiple interfaces was used.