Automated 3D Mapping of Submarine Tunnels

Advances in embedded computer architecture, digital signal processing, guidance and navigation, and digital sensing have made it possible to develop relatively compact instruments that can be used to create highly accurate three dimensional maps of channelized aquifers (flooded spring tunnels), bypass tunnels for dams, storm runoff networks, freshwater delivery tunnels, and other submarine features. One such implementation was recently designed and built for a National Geographic Society expedition to Wakulla Springs, Florida and was used to create the first three dimensional cave map. The Wakulla Digital Wall Mapper (DWM) consisted of a series of bulkhead-isolated cylinders measuring 324 mm diameter by 1122 mm in length and weighing 82 kg. The DWM contained five integrated subsystems including an onboard guidance and navigation system, a custom-designed sonar array, a digital user interface, a pitch/yaw control system, and a master integration and data management system. In all, eight onboard computers control the instrument. Drift in the Inertial Measurement Unit (IMU) was the major design concern since it directly affected the accuracy of the maps generated. A high-accuracy reference network of induction radio beacons was used to provide the necessary control. It was found that a time-based drift compensation algorithm, working between radio control points, provided a useful and simple approach to positional error reduction.