Submarine power and telecommunications cable projects involve a number of study phases including desktop/feasibility, route planning based on existing information, reconnaissance surveys to verify site conditions, and detailed final corridor investigations for design and government permits and requirements. Route location may be influenced by, but is not limited to; cost factors (size of the project/# of cables, route length), seabed conditions (hard bottom, sensitive habitats, adverse features such as large bedforms), oceanographic factors (water depths, currents, storms), environmental conditions (possible impacts to sensitive habitats, vessel traffic, fishing issues), government jurisdiction and requirements (state-federal waters), and overall risk assessment considering all pertinent factors. Nearly every type of marine survey system presently in use for the offshore industry will be utilized over the course of a cable project to help acquire the scientific information needed to support all phases of the work.
For the San Francisco PG&E power cable, a series of field programs were completed for each phase of the project, ultimately culminating in a cleared linear route ready for cable installation. A full geophysical equipment suite was used to map the cable corridor (multibeam, side scan sonar, magnetometer, high and low frequency subbottom profilers) with geotechnical vibracorers collected to directly sample subsurface sediments and to correlate with the subbottom profile interpretations. An abundance of debris on the bay floor required diver investigations and clearance of some objects away from the route centerline. Precision underwater positioning provided accurate diver locations to correlate real time visual observations and ID of objects to the georeferenced sonar imagery of the bottom. A high level of confidence was attained in this manner that the route was indeed cleared, reducing the risk of damage to the cable during the installation process.