A Borehole Seismic Reflection Survey in Support of Seepage Surveillance at the Abutment of a Large Embankment Dam

Retrofitting existing dams for the installation of modern monitoring instrumentation requires confidence in one's knowledge of the dam's internal structures. In 2010, the operator of the Mactaquac Generating Station wished to install a fibre optic distributed temperature sensing (DTS) cable as close as possible to the sub-vertical contact between the concrete diversion sluiceway and the clay till core of the adjacent zoned embankment dam. Given a lack of detailed as-built drawings, a plan was developed to image the interface by GPR or seismic reflection surveying from a sub-parallel borehole, offset by approximately 1 m at surface and by an estimated 4 m at the dam's foundation, near 50 m depth. Seismic reflection imaging, although novel for this application, emerged as the favoured approach after the range of borehole GPR surveys proved inadequate, due to high electrical conductivity in the concrete. A very high resolution wall-clamping seismic tool, with piezoelectric source and eight receivers, was operated in the dry borehole at 60 cm increments. Two surveys with different tool orientations were conducted to favour the reception of either P-wave or S-wave reflections from the interface, although such reflections were obscured, in the shot records, by relatively slow surface waves travelling along the borehole wall. A relatively simple processing flow involving bandpass filtering, CMP (common midpoint) stacking, and mean trace subtraction was successful in revealing an interpreted S-wave reflection from the interface, having a dominant frequency near 7 kHz representing a wavelength of about 35 cm. Interference from residual surface waves and apparent scattering from concrete layers, rebar or other heterogeneities near the borehole was very significant, but near-agreement of the interpreted reflection with the interface shown on engineering design plans provided confidence to proceed with the installation of two monitoring boreholes estimated to lie with 50 cm of concrete/clay contact.