5. On the Fantail or Back DeckThinking About the Earth's Surface

Main Menu	5. On the Fantail or Back Deck - Viewing the Equipment
Expedition Menu 1. Welcome 2. The Idea 3. Boarding 4. Ship Tour 5. Back Deck 6. Preparing to Depart 7. Leaving 8. Meeting 9. Bay-Delta-Estuary 10. Navigating 11. The Survey 12. First Data 13. The Mosaic 14. Visualizing 15. Hazards 16. Disposal Site 17. Sediment Map 18. Compare 19. Future Studies 20. Final Meeting		Tools of Our Voyage Here are three of the members of the ship’s crew and several pieces of equipment on the back deck (or fantail) of the ship. On the lower left corner is a winch that holds the cable used to lower equipment into the water and on the right is a sub-bottom profiler (in yellow), which is towed in the water, behind the ship, and uses sound waves to create a profile of the seafloor and image of the underlying layers of sediment (we will be using this instrument on our next expedition).
	On the current voyage, we will be using a Simrad EM-1000 multibeam sonar system to create detailed images of the bay floor. As the name implies, a multibeam system sends out and receives multiple beams of sonar energy in the form of a sound wave that bounces off the bay floor and returns (echoes) back to the ship. By measuring the time for the sound waves to leave the multibeam system, travel to the bay floor, and return to the multibeam instrumentation, located on the bow (front) of the ship, the water depth across a wide swath of the seafloor can be determined. By sending out this signal time and again (every second in fact) we will be able to construct a detailed image of the bay floor over a wide area.
		Animation modified from NOAA's Office of Coast Survey
	Multibeam Sonar System The multi-beam system consists of an array of transducers, which are small elements that generate an acoustic pulse when subjected to an electrical voltage. Once generated at the transducer, the acoustic pulse generates a sound wave that travels down into the water, bounces off the bay floor, and travels back to the sea surface, where the arrivals will be detected by a second array of transducers in the multibeam system.
		The acoustic energy that bounces off the bay floor is subdivided into 60 beams, 30 for each side of the ship, providing a 150^o swath of the seafloor centered beneath the ship. So for every transmission of energy from the multibeam system, 60 measurements of the water depth across a swath oriented perpendicular to the direction of the ship are obtained -- 30 on each side of the ship. By measuring the time it takes for the sonar signal to leave the transmitting array, bouncing off the bay floor and returning to the receiving array, we can determine 60 measurements of the water depths as we will see later in this expedition. Image courtesy of USGS
Created By: Don Reed Dept. of Geology San Jose State University ©Copyright 2008 Last Updated on Sept. 15, 2008	What do we need to know prior to leaving the dock?