Integrating LiDAR and Sonar Data for the Costa Concordia Wreck

A 3.205Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE

In early 2012 Codevintec, an Italian technology supplier for the earth sciences and geomatics industry for over 40 years, received an unusual call from the Crisis Team of Vigili Del Fuoco, Italy’s fire and rescue service. Only two days before, the cruise ship Costa Concordia had run aground off Isola del Giglio in Tuscany with the loss of 32 passengers and crew. Now the Crisis Team was turning its focus from rescue operations to recovering bodies and preventing the hazardous fuel from leaking.

The Costa Concordia still posed many unknown dangers to the rescue workers. As the ship was never designed to rest on its side, there were fears that its superstructure might buckle and warp, trapping divers and opening fuel tanks into the sensitive ecological area around the island. Even worse, sea charts showed that the ship was resting precariously on an underwater slope where the seafloor dropped rapidly, but the Crisis Team could not see under the ship to determine how stable its position was and whether it was in danger of suddenly rolling deeper.

With all these unknowns, the Crisis Team decided that it needed a complete survey of the wreck, both above and under the water. For this purpose, it contacted Codevintec about using their ILRIS/SeaBat system, which is specifically designed for simultaneous above-water and underwater surveying from a marine vehicle.

Sensor FusionThe ILRIS/SeaBat System
The ILRIS/SeaBat is a watercraftmounted system that uses different sensors to capture both topographic and bathymetric data: a side-mounted Optech ILRIS terrestrial laser scanner captures above-water data while a RESON SeaBat multibeam echosounder collects bathymetry data. Both sensors are integrated with a position and orientation system.

Mobile surveying with the Optech ILRIS LiDAR uses a different approach than static surveying. In static surveys, the ILRIS uses two scanner mirrors to spread the beam vertically and horizontally across the sensor’s field of view in order to create the 3D image. For mobile surveying, however, the ILRIS is mounted to point off the vessel’s port or starboard side, meaning that the movement of the survey vessel itself provides the horizontal motion. In this approach the ILRIS only needs to scan vertically as the vessel moves alongside the target, which is in fact how most of Optech’s mobile LiDAR systems operate.

The Teledyne RESON SeaBat 7125 surveys the seafloor with a multibeam echosounder, which provides highdefinition 3D point clouds using sound waves rather than lasers. Mounted on the survey vessel’s keel, the 7125 transmits acoustic 165-wide, fanshaped pulses. The return pulses are received by an array made of a multitude of hydrophone elements divided by a beam-forming (or beam-steering) technique into 512 beams, each looking in a specific direction. The result is a line (or section) made of 512 points along the 165 swath across the vessel’s track. These sections are updated up to 50 times per second and, together with the vessel’s movement, provide a 3D point cloud "strip" very similar to the one obtained with the ILRIS.

Because the vessel is moving, it is important to keep track of its attitude at all times. The ILRIS/SeaBat is compatible with a variety of inertial measurement units, but for this survey Codevintec used a Teledyne TSS Orion INS provided by the diver’s squad of the Vigili Del Fuoco. The Orion accurately monitored the vessel’s pitch, roll, heave and heading up to 200 times per second, using three solid-state precision linear accelerometers and three ring laser gyroscopes (RLGs) to produce exceptional attitude and heading performance based on the effect of gravity and the vessel’s rotation and movement.

All these systems are brought together by Teledyne RESON’s PDS2000 data acquisition and analysis software, which provides both calibration and real-time data display. Calibration is critical. To boresight the system after installation, the user first measures the distance and angle between the SeaBat and the ILRIS, and then collects 4 survey lines in a square around an object that extends both above and below the water. Next, the PDS2000 overlays the point clouds from these lines on each other, and then the user simply adjusts (automatically, manually or interactively) the roll, yaw, and pitch values for the ILRIS and SeaBat until they are perfectly aligned with each other.

Some of the earliest tests for the ILRIS/SeaBat system were performed in La Spezia with the assistance of the Italian Hydrographic Institute, and later in Venice with the support of Italy’s National Research Council (CNR), where the system was used to survey the city’s famous waterways and buildings. These early surveys ironed out how the ILRIS and SeaBat would be integrated with the attitude sensors, and confirmed that the system could be used in water as shallow as 0.5 meters.

After these tests, Codevintec integrated and delivered similar systems to many customers, which helped refine and optimize the integration even further. As a result, when the Crisis Team called to ask if such a system was available for use, Codevintec was able to offer a proven and mature technology.

Emergency ResponseSurveying the Wreck
The main challenge at the Costa Concordia site was time, as the rescuers needed to know whether the wreck was in an unstable position on the seafloor as quickly as possible. Codevintec rushed the system to the island in pieces and installed it on a ship already at the scene, the instrument research vessel Nereide. Fortunately, Codevintec has a great deal of experience in installing its system in various platforms, and the components require only limited integration with the ship itself. As a result, the installation began at 10 a.m. on January 17th and the entire operation was completed by 3 p.m. on the same dayincluding measuring the sensor lever arms, boresighting the system, performing the survey, and presenting the final results in 3D to the Crisis Team of the Vigili Del Fuoco and Costa’s own crisis unit.

Time was also a challenge for the survey run itself. Other vessels were remaining close by to assist the divers in recovery operations, and the survey would require them to stay away from the wreck, interrupting their urgent work. To minimize this disruption, Codevintec surveyed the entire wreck in a single 360 circuit by the Nereide and let the workers get back to work as soon as it was done. A single run left little margin for error, so they kept a close eye on the real-time display in the PDS2000 software to confirm that the LiDAR and sonar data were performing properly.

In general, the data collected by the system was good. With the SeaBat’s wide sensor angle, Codevintec captured a wide swath of data from the seafloor and ship in one pass, including plenty of data from underneath the ship itself. The smooth metal hull of the Costa Concordia was quite acoustically reflective, causing some of the SeaBat’s sonar pulses to be lost at first, but adjusting the sensor’s settings enabled Codevintec to collect a sufficient number of points.

The LiDAR data from the ILRIS was also good, but the position of the wreck created another challenge. Since the ship sank only a few tens of meters from the shore, the Nereide was forced to move fairly close to the wreck when surveying its landward side. This meant that the upper parts of the superstructure were above the ILRIS field of view (FOV). Normally this is solved by installing the ILRIS in a tilted position so that its full 40 FOV is aimed above the water. However, since Codevintec had little time at the wreck, they decided to capture the top of the superstructure from a static scan from the island. This was later performed successfully.

The entire mobile survey took a mere 5 hours from installation to the final report, and involved minimal disruption of the recovery efforts. Of course, a single survey can only tell you so much about a changing situation. With the first scan complete, the Crisis Team now knew where the wreck was lying. However, the team also needed to monitor the wreck constantly to determine whether it was moving from its surveyed position or deforming under stress.

To monitor the site continuously, Codevintec volunteered the services of the ILRIS again, this time using it in a static setup to ensure the best possible precision. Fortunately, switching the ILRIS from mobile mode to static mode is easy: Codevintec simply removed it from the boat, installed it on a tripod, and set it on dual-mirror mode.

Choosing a survey point now presented a challenge, as the shore near the wreck was still being used by the rescue crew, which might obstruct the sensor’s view. In response, Codevintec set up the ILRIS on a rocky hill on Isola del Giglio, so that it could survey the wreck over the top of the recovery efforts. This outcrop was several hundred meters away from the wreck, but because the ILRIS is an HD model with the Extended Range option, it easily captured data from that distance.

Survey ResultsIlluminating the Disaster
The Crisis Team was very impressed with the survey results, which provided an accurate view of the position and shape of the wreck and helped the Crisis Team to determine whether the wreck was in danger of rolling over. It also appreciated the fact that Codevintec was able to respond to their request so quickly and perform the entire survey with only a single pass around the ship. In the end, this project was a very successful exercise for the ILRIS/SeaBat system and provided a valuable tool for the recovery operation following this tragic event.

Dario Conforti has a degree in Urban Architecture from the Polytechnic University of Milan. With more than 11 years experience in laser scanner technology, he joined Optech in early 2006 as worldwide technical support and from 2012 as Sales Manager for Europe and Africa.

A 3.205Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE

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