High-Resolution 3D Laser Imaging Advances to Subcentimeter Precision
Authors: Wilson Adam 
Published: February 2015

This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 23006, ’SS MTS: Subsea Monitoring - High-Resolution 3D Laser Imaging for Inspection, Maintenance, Repair, and Operations,’ by C. Embry, M. Hardy, and B. Nickerson, 3D at Depth; N. Manning, CDL; D. Goodyear, UTEC Survey; and D. Richardson, SPE, and J. Pappas, SPE, RPSEA, prepared for the 2012 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed.
3D laser imaging is a powerful data-collection system that provides 3D information for a specific region of interest. Developing the technology to provide high-definition subsea laser imaging enables the deepwater industry to use the current state of the art in 3D metrology and related best practices developed for the terrestrial market. Terrestrial laser scanners commonly produce centimeter spatial and range accuracy at the several-hundred-meter range. Because of the absorption of water, realizable deepwater systems are limited to tens-of-meters range, depending on the target and water conditions. Here, subcentimeter accuracy from greater than 6 m is demonstrated for an underwater laser system.
Introduction
Sonar systems have been the main tools for measurement and survey for many years and have become very sophisticated. Many companies have developed high-resolution multibeam sonar systems that can produce angular resolutions as small as 0.2°. However, the resolution of these systems is fundamentally limited by the underlying physics; a wave-length of light is more than 108 times smaller than ultrasonic wavelengths and can achieve angular resolutions of less than 100 Μrad (0.006°). Therefore, light-based sensors inherently have higher spatial resolutions than sonar systems.
The underwater 3D laser sensor presented here has the fundamental advantage of providing subcentimeter precision from greater than 7 m, a capability that, to the authors’ knowledge, has not been demonstrated previously.
The Sensor
The optical head [laser imaging unit (LIU)] consists of the transmitter subsystem (laser and accompanying optics), the receiver subsystem (detector and accompanying optics), and the scanner. These subsystems can be optimized for different operational scenarios and environ-mental conditions. For this prototype system, we used a diode-pumped, passively Q-switched, Nd:YAG laser with an external nonlinear crystal to perform the frequency doubling to 532-nm laser light. The receiver is a high-speed silicon photo diode, and the scanning subsystem is a two-axis galvanometer scanner to allow programmable scanning in two orthogonal directions. Along with the LIU are multiple electronic control systems. These include the control subsystem for controlling the LIU and monitoring-system health; the central processing unit and bus for general system control and operation; the data subsystem for data collection, transformation, and processing; the data-storage system for housing data locally; the power and environmental monitoring system to convert the host-vehicle power to the various voltages required by the sensor; and the input/output (communication) subsystem. All of these systems are housed within the subsea sensor (Fig. 1). The prototype sensor is 8.2 in. in diameter and 28 in. long and weighs 76.1 lbm in air.