subbottom
profiler

precision
navigation

New archaeological techniques in the deep ocean set an agenda for technology development. In the spring of 1998, David Mindell and his research group built an instrument to project a narrow sonar beam into the seafloor and "see" down into the mud (in technical terms a high-frequency, narrow-beam, sub-bottom profiler). This data, when combined with computerized control and mapping, allow archaeologists to record and replay a "virtual excavation" of a wreck site, that is a three-dimensional model, removable in layers, all in a computer – without ever touching the wreck. (Update 2003: We have updated the sensor with new electronics and signal processing and look forward to the results of our Summer 2003 expedition.)

Sub-bottom profilers have been used for a long time in geology and other oceanographic applications, but usually at comparatively low frequencies (2-20kHz) and with rather wide beams (20-30 degrees). This device has a much higher frequency (150kHz) and a very narrow beam (3-5 degrees). While it doesn't penetrate nearly as deep into the mud as its lower-frequency cousins, the narrow beam allows the instrument to make detailed images which can depict small, buried features.

This device was successfully used for the first time on two Phoenician shipwrecks from the 8th-century B.C. off of Ashkelon, Israel, in June of 1999.  See the story "MIT technology helps map ancient Phoenician shipwrecks" in Tech Talk, July 14, 1999. Also see a National Geographic story about the expedition overall, "World's Oldest Deep-Sea Shipwrecks Found."

The 150kHz transducer for the profiler was fabricated by Marine Sonic Technology Inc. in Gloucester, Virginia. Then the MIT group added processing electronics, a computer, and a set of interface devices to run the instrument from JASON, the remotely-operated-vehicle run by the Woods Hole Oceanographic Institute.

This view shows the transducer on the bottom of Jason as the vehicle is lifted over the side to be deployed. The sub-bottom transducer is the large gray circle in the middle of the photograph. (The instrument to the right rear with the lens is an electronic still camera, and that with the green potting around the transducers is Louis Whitcomb's acoustic Doppler current meter, used to precisely measure Jason's speed and direction).

The circuitry for driving the transducer, digitizing the data, compressing it, and sending it up Jason's fiber-optic telemetry link. The computer is a TT8 made by Onset Computer, and is a 68332, basically the same chip that runs a PalmPilot. Note the green endcap for the pressure housing and the gray transducer in the background, set up on the lab bench for testing.

Deep Water Archaeology Research Group Massachusetts Institute of Technology 77 Massachusetts Ave. Rm e51-194 Cambridge, MA 02139