oa Shallow Seismic Reflection Applied To Quaternary Paleochannels

A high resolution seismic reflection survey was conducted<br>at Taylors Island, Maryland with the objective of identifing buried<br>paleochannels of the Susguehanna River. One 2.5 km six-fold CDP<br>reflection profile was acquired using a portable Bison 8012A<br>seismigraph, a 10 lb. sledge hammer/aluminum plate source, and 12<br>single accelerometers as receivers. Reflections from interfaces<br>to depths of 100 m were desired in order to observe the<br>relationship between incised Quaternary paleochannels and the<br>underlying Tertiary marine sediments. The data were collected<br>using an end-on spread configuration with a 25 m source to first<br>receiver offset and a 5 m receiver spacing. Profiles obtained from<br>processing of the data clearly indicate the presence of one or more<br>paleochannels in the subsurface that were cut during a low stand<br>of sea level. Channel margins are apparent due to the undulating<br>nature of the unconformable reflecting interface that is observed<br>to truncate older concordant sub-horizontal reflectors below.<br>Channel fill reflectors representing sediments deposited during a<br>subsequent high stand of sea level are generally horizontal and<br>concordant with underlying seismic reflectors except where they<br>onlap at the channel margins. The shallowest fill reflector<br>detected is at 10 m depth, whereas a channel depth of 53 m is<br>calculated assuming a velocity of 1420 m/s. A spectral analysis<br>of selected traces indicate a dominant frequency of approximately<br>125 Hz, with frequencies of up to 200 Hz not uncommon. The<br>theoretical vertical resolution of 1.8 m is possible using the l/4<br>wavelength criterion.