Continuous Seismic Profiling (CSP) at Callahan Mine Site: May 2017 to July 2018

In May 2017 and July 2018, continuous seismic profiling (CSP) surveys were collected in Goose Pond at the Callahan Mine site. The chirp frequency CSP tool was used in the deep water. A tuned transducer (dual-echo sounder) was used in the shallow water, and the data are served separately but within this data release (See CSP-DSO). CSP methods use an acoustic sound source that travels through the water column, reflects off the bottom and sub-bottom layers and is received at the transducer. Using a water column velocity, the two-way travel time can be converted to distance. CSP methods provide the depth to water bottom, and when sufficient signal penetration can be achieved, CSP can be used to delineate the depth and topography of the subbottom layers and/or the bedrock surface. In a limited number of places, the bedrock surface was observed in the CSP record, creating a discontinuous and sporadic image of the bedrock surface. Some profiles exhibited multiple stratigraphic layers. The interpretation of subsurface in the seismic record generally matched depths to till or bedrock observed in core data (when/where the signal penetrated the subbottom). However, in many places, CSP surveys were adversely affected by a highly reflective water bottom, which caused strong multiples in the seismic record and limited to no depth of penetration. These multiples are attributed to the signal reflecting off of an acoustically hard layer caused by entrapped gas (methane) in the sediments, compacted sediments, or bedrock. The returning signal reflects off of the water/air interface, returns to the acoustic hard layer and reverberates between these two interfaces.

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