Posted by 3D at Depth Pty Ltd on 03/07/2017

Using Underwater LiDAR for Cave Mapping

Using Underwater LiDAR for Cave Mapping

Mark Hardy of 3D at Depth, introduces the underwater laser scanner which, by its use of a green laser, is able to provide similar results to those which infrared laser scanners can achieve in air.

Cavers are now familiar with the use of large tripod-mounted laser scanners for surveying large cave chambers – see, for example, (Walters, 2016) – and handheld laser scanners for surveying cave passages – see (Williams, 2014) and (Cadge, 2017). However, the infrared beam transmitted by these scanners penetrates water very poorly. Cavers have occasionally used sonar to achieve similar results in flooded caves (Pease, 2000) and a recent article explained this technology (Dobson, 2016). Here we present an alternative method of detailed underwater surveying – the use of a special type of laser scanner, designed exclusively for this environment, and which could find applications in cave surveying.

Introduction

LiDAR – Light Detection and Ranging – has become the predominant tool for a variety of mapping and survey applications. The technology is based on the concept of sending a beam of light from a transmitter, striking an object and then precisely measuring the time that it takes to return. There are several implementations of LiDAR on the market including Time of Flight, Pulsed and Geiger systems. Today LiDAR has been adopted by a variety of markets including autonomous vehicles, which use special software tools to determine their location and direction based on surrounding objects.

In 2009, the Research Partnership to Secure Energy for America (RPSEA), in conjunction with the National Energy Technology Laboratory, awarded a grant to 3D at Depth to bring their underwater laser scanning concept from the lab into the field. In 2014, 3D at Depth successfully com-mercialised their technology by launching the SL (subsea LiDAR) products. Since then the SL1 and SL2 underwater laser scanners have been used on over 100 projects worldwide and have become the standard in high resolution underwater data collection.

Applications

Primarily used in the offshore oil and gas market, the SL systems have also been implemented for scanning inland dams, shipwrecks including the USS Arizona and, in a recent project with the BBC, have scanned ancient underwater ruins in Naples Italy. In 2014, the US Park Service used the SL2 to scan a remnant of a sunken 1950’s era row boat in Yellowstone Lake (see picture).

“We attempted to bring the technology, datasets and workflows from topside laser scanning to underwater environments.” says co-founder and CEO Carl Embry “In order to allow rapid market adoption, clients needed to have a short learning curve. We worked closely with our partners to make sure that they could collect and process our datasets with the same tools that they use for their topside laser scanning projects.”

The Technology

Although the high-level concepts are similar to topside laser scanning, the technology for measuring the speed of light though multiple mediums required specialized optics, receivers and patented algorithms developed by 3D at Depth. The light source is green, which provides the best performance in water at 532nm, and the temperature, salinity and pressure of the environment are also measured and used to determine the associated range or distance from the scanner to the target. An internal scanner, consisting of dual-axis mirrors, direct the light in azimuth and elevation, providing the ability to generate a 3D point cloud of the area.

During the design of the system there were trade-offs between size and performance, which resulted in a scanner field of view consisting of a 30° × 30° swathe. To expand the field of view, the SL systems rotate on specially designed pan and tilt motors which provide a 360° capability. Due to the unique receiver, the scanner can also collect multiple returns for each pulse of light and filter the particulates which are visible in most underwater environments. In clear water environments, the scanner can collect data up to 40 metres away but this is reduced as the amount of particulates increases and water clarity decreases.

There are two types of underwater laser scanners on the commercial market; triangulation and time-of-flight. Triangulation systems utilize a laser-profiling beam and a camera / CCD-based receiver. Although triangulation systems offer very high resolution at shorter ranges (<2m), their accuracy degrades exponentially with increasing range and also provides only a single return for each laser pulse – not as useful in turbid water conditions. During thscan positions). Multiple setups allow for the mapping of large areas and for areas where a different line of sight is required. To register the scan positions into one coherent model, specialised spheres were developed with coatings that are optimized for the green laser. Planning the collection requires an understanding of the system’s range in the specific environment and will include the placement of the spheres in overlapping scan positions. Once the data has been collected, the scan positions can be registered together using these coincident target spheres. The model can then be further enhanced using cloud-to-cloud registration techniques called iterative closest point (ICP). ICP allows the points from one scan position to align with points from adjacent scan positions and utilise the data from the e57 point cloud format.

Caving Applications

The SL2 could be an effective method of mapping caves and would have similar workflows to recent cave mapping projects. Deploying an SL2 on a diver frame requires an umbilical to a topside computer that would control and receive the resulting datasets as well as providing a method for quality control. Spheres could be mounted on the cave walls or placed on the bottom using stands. The SL2 would then be repositioned by divers until the full area of interest has been collected.

Perhaps this technology could be valuable for unlocking the secrets hidden beneath the water and allow researchers to map previously undocumented areas and bring laser scanning to new depths.

For information on the SL systems, please visit the 3D at Depth website, www.3DatDepth.com Systems are available for lease on a per-project basis.

References

Cadge, Stuart (2017) Cave Surveying with the GeoSLAM ZEB-REVO, CREGJ 87, pp. 3-5.

Dobson, Christian (2016) Introducing Sonar Technology as a Tool for Underwater Cave Surveying, CREGJ 94, pp. 20-23.

Rabson, John & Rabson, Rosy (2000) Digital Wall Mapping at Wakulla Springs, Florida, CREGJ 39, pp. 13, 14.

Walters, Richard (2016) Laser Scanning the World’s Largest Cave Chambers, CREGJ 94, pp. 6-10.

Williams, Emily (2014) Handheld Laser Scanning: a Radically New Approach to Cave Surveying, CREGJ 86, pp. 21-24.

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