SEISMIC INVESTIGATION OF FAULTED, SEDIMENTARY BEDROCK AT THE FORMER NAVAL AIR WARFARE CENTER, WEST TRENTON, NEW JERSEY

At the former Naval Air Warfare Center (NAWC) in West Trenton, New Jersey, faults in the sedimentary bedrock were investigated with two seismic refraction surveys because such faults affect the flow of groundwater and dissolved contaminants. The refraction surveys were conducted in open land that is adjacent to the NAWC site. The surveys used S-waves to probe the soil and bedrock because these waves are particularly sensitive to faults and fractures. The result of each survey was a seismic cross section showing the velocity of the S-waves within the ground as a function of horizontal distance and elevation.

In one seismic cross section, the S-wave velocity increases from about 100 m/s at the ground surface to about 450 m/s at about 5 m depth, and this interval is interpreted as soil, wind-borne sands from the last glaciation, and highly weathered bedrock. Below this layer, the S-wave velocity generally increases from about 450 m/s to greater than 1620 m/s at about 25 m depth, and this interval is interpreted as sedimentary bedrock. The velocity in the bedrock also varies in the horizontal direction and is especially low in a 35-m wide zone near the middle of the cross section. This low-velocity zone is collinear with a fault observed in rock core from a nearby well and with a fault observed in rock cores from several wells within the NAWC site. Therefore, this low-velocity zone is interpreted as a fault; furthermore, this fault, the fault observed in the nearby well, and the fault observed in the NAWC wells are interpreted as the same fault. This fault affects static water levels within the NAWC site and, hence, affects the flow of ground water and dissolved contaminants. The other cross section is similar to the previously discussed cross section, and it too contains a low-velocity zone whose significance is currently being investigated.