Today’s technology records depth from land to deep waters accurately and efficiently.
More than 90% of the world’s trade is carried by sea, according to the International Maritime Organization. Therefore, accurate and up-to-date charts of coastal waters for ship navigation are vital for world commerce. The current expansion of the Panama Canal, which will double its capacity and allow larger ships to transit, is further increasing the need for depth data—as ports compete for the ability to accommodate the new “Panamax” ships being built to take advantage of the canal’s enhanced dimensions.
Population growth in coastal areas and sea level rise due to climate change are driving the need for bathymetric data for planning and emergency management. The advent of unmanned aerial vehicles (UAVs) has accelerated the ongoing effort to miniaturize bathymetric sensors by reducing their size, weight, and power (SWAP) requirements. These factors, in addition to technological advances in the sensors’ hardware and software, are bringing new tools to the ancient science of bathymetry.
For centuries, mariners recorded water depth using nothing more than a lead line of sufficient length, a compass, a sextant, and a rudimentary nautical chart. This was such a time-consuming process, however, that they could only perform it for a tiny percentage of the world’s oceans and coastlines. Today’s technology makes the process not only more accurate, but vastly more efficient as well.
In deep waters, depth data is collected using huge multi-beam echo sounders that operate at very low frequencies. As the depth decreases, smaller devices are used that operate at higher frequencies and, therefore, higher resolution. However, very close to shore, the efficiency of these devices drops dramatically, as the cone of their sound signal is cut off by the slope of the shelf. This is where airborne lidar sensors become a much more efficient means of collecting depth data.
Teledyne, a U.S. manufacturer, builds devices needed to measure water depth, from the deepest ocean trenches all the way to shore and onto land, and writes software to process the data. A conversation with them about their products provides an overview of the unique challenges and some new solutions using bathymetry.
Deep WaterEquipment: 12 KHz multi-beam systems
In the deepest waters, hydrographers use multi-beam systems that operate at a frequency of about 12 KHz, typically measure about 10 meters by 10 meters in a cross or T configuration, and weigh a few tons, says Douglas Lockhart, chief hydrographer for Teledyne RD Instruments, one of Teledyne’s divisions. They can reach even the bottom of the deepest trenches in a single pass.
However, the beam’s acoustic footprint will be large by the time it reaches the bottom, so the returns will be at low resolution. “The beam may start out as half a degree or even a quarter of a degree,” Lockhart points out, “but by the time it goes, say, 11 kilometers it is going to be insonifying [flooding with sound waves] an area the size of a football stadium when it hits the sea floor.”
It is relatively easy to use a single-beam echo sounder to bounce sound off of something and get a fairly accurate value, as consumer-grade sounders do, Lockhart explains. “It gets harder when you have the multi-beams and you start spraying sound all over the place because the boat is moving and you are trying to correct for all that motion. For charting, you have to apply motion and position and a tidal datum and a sound velocity correction.”
Another way to measure the depth of the deepest waters is by using autonomous underwater vehicles (AUVs). Some manufacturers are now making AUVs with a 6,000-meter range, enabling them to measure the shape of the ocean floor with great accuracy, while using pressure sensors to know their own depth in the water.
“I’ve been on some deep dives,” Lockhart recalls, “and pretty much all you have to tell your depth is a gauge that looks a bit like a clock.
It will have a dial for PSI and another one for depth. Forget all the nonsense; you are within ten feet or so of that depth. For a lot of work, that is enough.”
Simply adding the AUV’s depth in the water and its distance from the ocean floor will yield the depth of the water in that location. However, when extreme accuracy is required, hydrographers will also take into account the barometric pressure of the air above the surface of the water, the density of the water (which changes with its salinity), and even the local gravity.
Intermediate DepthsEquipment: 50 KHz – 70 KHz multi-beam systems
In shallower waters, the 12 KHz systems are replaced by systems that operate somewhere between 50 KHz and 70 KHz, says Lockhart. “They will work for you from, say, 3,000 meters to fairly shallow waters. You can get pretty good-looking data in 100 meters or less.” As the depth continues to decrease, the frequency of the devices continues to increase—to 100 KHz, then 200 KHz, and eventually to 450 KHz or 500 KHz, for very high-resolution mapping.
“The higher frequencies are going to extinguish pretty quickly, so you are not going to be able to map out past 50 meters or 100 meters very efficiently with them,” says Lockhart. “With all of these systems, if you go out too deep the swath gets narrower and narrower, until they are acting like single-beam echo sounders if it gets really deep.”
Equipment: Airborne lidar systems using green lasers
Single-beam and multi-beam sonars are efficient for depths greater than about 25 meters. “When you come in shallow, you start becoming less and less efficient, because the swath width is a function of water depth,” Lockhart explains. “At some point, there is a cross over, where airborne green laser lidar systems become more efficient to operate. Where land survey methods end, on the beach coming down, and ocean survey methods end—where the breaking waves are and it is really shallow—there’s a ‘white stripe’ of data that is pretty hard to get. That is where the airborne lidars can fill in. In many cases, they are the most efficient solution to survey in that area. In some cases, they are the only solution, due to safety and for other reasons.”
The high frequency sensors used in shallow waters are much smaller than their deep-water counterparts. “We make one that is about the size of a toaster,” says Lockhart.
CoastlineEquipment: Airborne lidar
When performing airborne lidar surveys of coastlines, the aircraft’s course and the width of the sensor’s swath depend on the area being surveyed and the target data density. For an aircraft flying at about 400 meters, for example, the sensor swath will be on the order of 100 meters.
“Depending on the shape of the coastline, the surveyors will set up their survey pattern to be as efficient as possible,” says Lockhart. “If they are just trying to fill in an area that is linear along the coastline, they will be flying parallel to the shore and then out to a distance where they either get down as deep as they can view with lidar, or they cover over the data from a multi-beam.”
Many variables can come into play with airborne lidar. For example, the exact details of the flight pattern may depend on the position of the sun, as surveyors try to give their laser the best chance they can given the time of day.
Water and Land
Teledyne manufactures two sensors that can be used to map both water depth and land elevation along coastlines: the Coastal Zone Mapping and Imaging Lidar (CZMIL) system, launched in 2012, and the much smaller and less expensive Titan, to be released this month at the Intergeo conference and trade show in Berlin.
The CZMIL “is an innovative airborne 3D environmental coastal mapping system that combines active and passive sensors with the complete workflow, providing seamless information from land to shallow water to deep waters up to 80 meters,” says Max Elbaz, the president of Optech Inc., a company owned by Teledyne. It can also capture depth information from turbid waters and muddy bottoms.
The CZMIL comprises three different devices. It has a lidar sensor that combines a green laser, which penetrates the water, and an infrared laser, which does not penetrate the water and returns the elevation of the land, each with its own separate receiver. It also has an RGB digital camera and a hyperspectral camera.
“We have combined everything into a true co-registered sensor fusion,” Elbaz says. “We also developed the full workflow, to capture simultaneously all the information from all of these different types of sensors and then process it all at the same time. It is called HydroFusion.”
The Titan has two to three wavelengths, depending on the user’s preferred configuration, explains Michael Sitar, Optech’s business manager for airborne survey products. “It will allow us to do some shallow water depth mapping but also maintain the high accuracy associated with conventional surveying on land.”
Additionally, the new sensor will maintain the same sampling rates for all the different wavelengths, thereby yielding the same resolution. “So, your point density is similar, irrespective of what wavelength you choose,” says Sitar. “As you move inland, you will probably switch to a 1-micron-only sensor for topo mapping, and we would expect to have the same resolution as that of our coastal sensors.”
“There is a class of sensors that have mapping capability both on land and in shallow water depth and have a price point that is far below that associated with the larger, more powerful bathymetric sensors that AHAB and Optech have fielded in the past,” says Sitar. “They are more associated with shallow water surveys, because they don’t have that stronger laser power.”
Optech has been a leader in lidar and imaging on land for 40 years. For working with hydrography, their Orion platform “is a single scanner compact sensor,” Sitar explains. “That platform is really about integration with peripheral products, so it is for creating multi-sensor configurations.”
When to Switch
When hydrographers switch from one device to another depends largely on the application and on the slope of the continental shelf. “The U.S. Army Corps of Engineers (USACE) only needs to provide up to 1,000 meters offshore,” Elbaz points out, “so they use a combination of airborne lidar and sonar systems. At depths above about 25 meters, some agencies will use multi-beam sonar; at depths below 25 meters, you need to use an airborne lidar system, for two reasons: efficiency of collection and hazards.”
Covering the Whole Range
All the depths, from the deepest trenches all the way to shore, can be adequately measured with, at most, four sensors, some of them multi-frequency systems, according to Lockhart. Teledyne manufactures all of these different types of sensors. In addition to single-beam and multi-beam echo sounders, some of which have a 3D capability, it makes the motion reference units that measure the motion of vessels and sound velocity probes. The company’s Reson division also writes a full software suite called PDM that performs full-motion correction, plotting, charting, and dredging sections.
Optech pioneered airborne bathymetric sensors, according to Elbaz. “Since the early 1980s, we built sensors for the Canadian Hydrographic Service, the Swedish Defense Research Agency, and the U.S. Defense Advanced Research Projects Agency (DARPA),” he recalls. “In the 1990s, we introduced the Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) system, then a couple of major improvements to it. Everything culminated with the introduction of the CZMIL topo-bathymetric system to the USACE and the U.S. Navy. USACE wanted to seamlessly capture topographic and bathymetric data for its National Coastal Mapping Program, which is chartered to map from 500 meters inland to 1,000 meters offshore.”
“Optech is really now both a lidar company and a camera company,” Sitar points out. We are positioning our products as complementing technologies for enhanced information extraction. So, very often the sensors we design now are really integrated sensors, with both passive and active imaging capabilities.”
Teledyne, which is a publicly-traded corporation, took a 51% ownership position with Optech a couple of years ago. “Teledyne has technology and systems that are very complementary to Optech’s airborne lidar systems,” says Elbaz. “That is the reason they acquired Optech. For mapping coastal areas, we now have the capability to seamlessly merge information from airborne lidar with information from sonar systems. Then you have very seamless information from land, to shallow water, to deep water. Our airborne system is used now for nautical charting. The industry has accepted that airborne lidar systems provide excellent accuracy and precision and in some instances are as good as, or even better than, sonar data.”
Applications and Drivers
“Most of the work that is done in deep water is either research and geology-based or for cable surveying,” points out Lockhart. “All the optical cables that wrap around Earth now are surveyed-in with deep water systems. The oil industry uses deep water systems as part of their exploration tools.
“As you come in shallower, you start seeing applications for habitat studies,” he continues. “Where are the fish living and what is happening to their habitat? That is kind of a blend of geology and biology. Then, as you come in even shallower, you start seeing work done for infrastructures—ports and harbors—and for navigation. Once you get to 30 meters and shallower, navigation and dredging become the largest drivers for everything. Most of the survey work in coastal areas has a navigational aspect to it.”
In addition to technological advances, a few key concerns are driving the development of bathymetric sensors. One is the under-keel clearance of large ships. The expansion of the Panama Canal has changed what’s called “Panamax,” the size of the biggest ship that can fit through the canal. Marine and port authorities want to make their shipping channels deep enough to accommodate the new Panamax ships, but not any bigger than they have to—especially on the East Coast of the United States where some of the channels have hard rock bottoms, unlike the mud bottoms on the West Coast. Under-keel clearances can be as little as 20 centimeters or even 10 centimeters. The stakes are very high because when a cargo ship can load additional cargo that will sink it, say, another 10 centimeters in the water, the difference in dollars can be huge.
With regards to shallow water bathymetry, one driver is the need to measure depths in turbid waters near urban areas, which represent about 80% of the world’s coastal areas. “More than 40% of the world’s population and 75% of all urban centers are in or near the coastal environment,” says Elbaz. “Until now, with airborne systems, you could only capture information from very clear water.”
The other driver in shallow water is the need to understand near-shore habitats, which is critical for management decisions, disaster planning, monitoring environmental impacts, and the challenges from growing populations and rising sea levels.
Beyond bathymetric surveys of ocean depths and topographic surveys of land elevation lies the opportunity to measure and model dynamic phenomena, such as waves and their impact on the surrounding environment. “There hasn’t been a lot of research on modeling waveform movement using lidar techniques,” says Sitar. “It’s an area that we are exploring.”