An integrated waterborne seismic-electromagnetic acquisition system and its application in bedrock investigation for cross-river tunneling

Construction of cross-river tunnels requires an understanding of the geology beneath the riverbed, particularly the solidity of the bedrock that may often be compromised by the weathering process. High-efficiency geophysical technology is demanded before drilling and other intrusive verification. We develop an integrated seismic-electromagnetic acquisition system for towed waterborne surveys by combing the single-channel seismic and towed transient electromagnetic (FloaTEM). The system consists of one towing boat and two floating boats connected and traveling in a line. The front deck of the towing boat houses the transmitting and receiving unit of the seismic system, and the seismic sensors are attached to one side of the boat below the waterline. The back deck contains the boat engine and the control unit of the FloaTEM system, whose transmitter loop and receiver coils are mounted in the two floating boats behind the towing boat. The system was tested at a section of Shunde Waterway near Guangzhou, China. The entire survey of about 7.6 line-km was finished in about 1.3 hours at a speed of 6 km/h. The seismic data and EM inversion model are highly consistent in delineating the three major interfaces below the water surface: water-sediments, sediments-weathered bedrock, and weathered-unweathered bedrock. In addition, the EM method is particularly useful in highlighting the contrast between the weathered and unweathered bedrock because of the difference in water content. The integrated survey revealed that the weathered bedrock layer thickens from the north to the south along the planned tunneling line, which is also favorably verified by the drill holes on the river bank of both sides.