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High-Resolution Echosounder Survey In Villefranche Bay
Conducting a Bathymetric Survey in the South of France Using a SONIC 2024 Broadband Multibeam Echosounder

By Alexandre Dano
G'oazur Laboratory
Villefranche-sur-Mer, France

Villefranche Bay—3.5 kilometers at it longest and 1.5 kilometers at its widest—was first bathymetrically mapped in the early 20th century, but it was not until the 1950s that scientists created the bay's first modern bathymetric map. It was then that Claude Lalou, a scientist working at the Villefranche-sur-Mer Oceanology Observatory (OOV), used a French navy single-beam echosounder to build an accurate bathymetric map of the bay.

Surprisingly, though the population of scientists working in Villefranche-sur-Mer had grown, researchers had never drafted a new bathymetric map of the entire bay. So in January 2010, Géoazur Laboratory, a seismology and geology laboratory with its marine geosciences division located at OOV, teamed up with Cadden (Nantes, France) to repeat Lalou's survey using a modern multibeam echosounder.

Echosounder survey results from Villefranche Bay, projected on satellite imagery from Google Earth. (Photo courtesy of Alexandre Dano/Géoazur)

Organizing the Survey and Workshop
Scientists working at OOV had been keen on getting a new, relevant bathymetric map of Villefranche Bay at a higher resolution. OOV scientists organize daily excursions for instrument tests and data collection; a good knowledge of the bay's seafloor allows the scientists to choose sample locations and instrument launches with greater accuracy.

In November 2009, Géoazur invited Cadden personnel to conduct a workshop with an R2Sonic (Santa Barbara, California) SONIC 2024 broadband multibeam echosounder, with two days to survey the bay and one day dedicated to presentations and workshops. The goal was to cover an area at least the size of the 1950s survey during the two-day timeframe. The survey and workshop were scheduled for January, a month when the bay is not crowded with cruise liners.

Survey Preparation
On the first day of operations, the team began by mounting the echosounder equipment on OOV's 8.5-meter-long ship, La Sagitta. Installing the hardware, the ultrahigh frequency and global positioning system (GPS) antennae, the inertial motion unit (IMU) and the ground GPS real-time kinematic (RTK) station went quickly, but it took the rest of the day to make the echosounder antenna base fit with La Sagitta's bow. This operation required pieces of wood, a small boat for installation and a bit of patience to check the levels. Once all of the equipment was installed, the team spent the morning calibrating and fitting the instruments on the boat. They then took a velocity profile using a Valeport Ltd. (Totnes, England) sound velocity profiler, and the survey began in the afternoon.

Echosounder Capabilities and Results
The SONIC 2024 echosounder has several features of interest for this survey, including an adjustable frequency from 200 to 400 kilohertz by steps of 10 hertz, a 60-kilohertz bandwidth, a ping frequency reaching 75 hertz, a 10° to 160° swath coverage and a practical electricity consumption of 50 watts and 220 volts.

The adjustable frequency is useful in regions with bathymetry like Villefranche Bay, where depths fall steeply from one meter to 360 meters within 500 meters of the coast. QINSy processing software from Quality Positioning Services B.V. (Zeist, Netherlands) was used to deal with these irregular profiles.

Surveying with the SONIC 2024 provided spectacular results, imaging in considerable detail the harbor's dike and a shipwreck located in front of the main beach at 18 meters' depth. A diving team managed to take some pictures of this 15.5-meter wreck, identified to be the diving support vessel Tiberiade, and the photos agreed with the echosounder results. A 15-centimeter-diameter fallen mast on the wreck's port side is clearly visible in the echosounder images.

The resolution provided by the echosounder ranges from two meters at 332 meters' depth to five centimeters at five meters' depth. Such high resolution requires a positioning system with proven performance. The Magellan (Santa Clara, California) Proflex 500 offers a maximum positioning error of two centimeters in RTK mode, and the CodaOctopus Products Ltd. (Edinburgh, Scotland) F185R+ IMU offers a maximum error of 20 centimeters.

(Above, left) Tiberiade's wreck on its port side, with its fallen mast visible in the rear. (Photo courtesy of Dominique Chassagne/Cadden)

(Above, right) Diving picture showing the front of the Tiberiade on the sea floor at 18 meters' depth. (Photo courtesy of David Luquet/OOV)

Even with such highly accurate instruments, there were some challenges to collecting high-quality data. When using a high-resolution echosounder mounted on a lightweight ship like La Sagitta, even a minor storm can have a major impact on data quality. While trying to go around Cape Saint-Jean Cap Ferrat east of the bay's entrance, a 1.5-meter swell altered the data reading. In response, the survey vessel had to come back around the cape to seek protection from a one-to-two-meter swell coming from the east.

By the end of the high-resolution survey, the team had extended the historical survey area to the south and to the west by reaching greater depths: 356 meters in 2010 compared to 250 meters in the 1950s. The 2010 survey covered a four-square-kilometer area.

Conducting the survey in Villefranche Bay, with its flat bottoms, steep slopes, rocks, wrecks, breakwaters, and deep and shallow areas, offered an additional benefit to Cadden, which had not yet had the opportunity to test the SONIC 2024 to its maximum depth. The diversified profile in the bay helped the company test the echosounder's performance.

Workshop and Collaboration
About 20 people attended the workshop, with all time slots booked. Participants went aboard La Sagitta for a tour of the bay to check how the echosounder and its auxiliary devices functioned.

The survey and workshop are a good example of what cooperation between a public laboratory and a private firm can achieve without either party losing independence. Gèoazur gained high-resolution bathymetric data of the bay to better lead OOV tasks, and Cadden made new contacts with customers in southern France.

The author would like to thank Gilles Dandec and Dominique Chassagne at Cadden; Jean-Yves Carval, David Luquet, Jean-Philippe Labat and Fauzi Mantoura at OOV; L'Alchimie and its crew; the author's colleagues Jean Mascle and François Michaud; and Virginie Fombaron, for writing and editing assistance.

Alexandre Dano graduated with a master's in geophysics at IPGP, the Earth Sciences Institute of Paris, and joined Gèoazur Laboratory in 2007 as a French National Scientific Research Centre engineer. He is in charge of bathymetric instruments, data processing and management, and he is the technical project leader for a new complete multibeam echosounding system.

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