Best Boats For Near-Shore Bathymetric Surveying Operations

Bathymetric surveys are used to measure the depth of a water body and map its underwater features. They are often used for marine construction projects, such as dredging or offshore energy, and can be conducted in both deep and shallow waters. When it comes to selecting the best boat for near-shore bathymetric surveying, a few options are available, including:

- Unmanned Surface Vessels (USVs): These are ideal for shallow water surveys and can access areas that are difficult or unsafe for traditional vessels. Examples include the CEE-USV™ and the BathyCat Survey Catamaran.

- Bathymetric Survey Boats: These are equipped with advanced technology, such as survey-grade sonars and RTK GPS, to provide real-time, accurate data. An example is the Kingfisher, used by Hunt Surveys Inc.

- Custom Vessels: For deep-water surveys in seas and oceans, custom vessels with multi-beam survey technology are typically used. These vessels use multi-beam echo sounders to send out a wide array of beams and calculate the distance to the seafloor.

Single-beam vs multi-beam surveys

Single-beam and multi-beam bathymetric surveys are used to map the ocean floor and collect data on water depths. The choice between the two depends on the specific demands of the hydrographic survey and the intricacies of the underwater landscape. Here is a detailed comparison between the two methodologies:

Single-Beam Surveys:

Single-beam echo sounders (SBES) are the pioneers of underwater mapping technology. They are straightforward, relying on a single transducer for transmitting and receiving signals. The beam's width is influenced by transducer dimensions and acoustic wavelength, with larger transducers yielding narrower beams. SBES is ideal for shallow-water surveys and straightforward tasks, offering high-resolution data in focused areas. It provides simplicity and precision in narrow tasks.

Multi-Beam Surveys:

Multi-beam echo sounders (MBES) represent a dynamic evolution, capable of scanning larger areas with greater finesse. MBES employs separate transducer arrays for transmission and reception, maximizing efficiency. It produces a fan-shaped transmission beam, narrow along the survey track and broader across it, allowing for a comprehensive picture of the underwater terrain. MBES offers superior coverage compared to SBES, but at the expense of resolution in individual beams.

Stability and Precision:

SBES may struggle with stability in turbulent waters due to its reliance on platform attitude. In contrast, MBES can adjust for platform motion, offering better precision in challenging conditions. MBES is particularly useful for complex underwater terrains where side lobe interference could skew results.

Efficiency:

While SBES can be faster in shallower areas, MBES excels in deeper waters and large-scale mapping projects. MBES has a wider swath reach, enabling efficient data capture over large areas and reducing the time required for data collection.

Applications:

SBES is well-suited for straightforward surveys, such as in shallow waters or when high directivity is required. It is a good choice for projects that require simplicity and precision in focused scenarios. On the other hand, MBES is ideal for intricate mapping tasks, such as in deeper waters or when comprehensive coverage is needed. It is often the preferred choice for modern hydrographic endeavours.

Acoustic Doppler Current Profilers (ADCPs)

ADCPs use a series of acoustic transducers that emit and receive "pings" of sound from different directions. These pings bounce off particles suspended in the moving water and reflect back to the instrument. The Doppler effect is used to calculate how fast the particle and the water around it are moving. The difference in frequency between the waves the profiler sends out and the waves it receives is called the Doppler shift.

ADCPs can be mounted directly on a stationary object like a mooring buoy or even directly on the seafloor. They can also be mounted to a moving vehicle, such as ships or unmanned underwater and surface vehicles. On large research vessels, the ADCP is permanently mounted on the bottom of the ship’s outer hull.

ADCPs are available in different frequencies. Higher frequencies, such as 300 kiloHertz (kHz), provide high-resolution data near the surface to a depth range of around 70 meters (230 feet). Lower-frequency ADCPs, such as 38 kHz, will provide lower-resolution data to a depth range of up to 1,300 meters (4,265 feet), depending on surrounding noise levels in the ocean.

There are several advantages to using ADCPs. They can measure small-scale currents and the absolute speed of the water, not just the relative speed of one water mass in relation to another. They can also measure a water column up to 1000 meters long.

However, there are also some limitations to ADCPs. High-frequency pings yield more precise data, but low-frequency pings travel farther in the water. Therefore, a compromise must be made between the distance that the profiler can measure and the precision of the measurements. Additionally, ADCPs set to "ping" rapidly can quickly drain batteries. If the water is very clear, the pings may not hit enough particles to produce reliable data. Bubbles in turbulent water or schools of swimming marine life can also cause the instrument to miscalculate the current.

Sub-bottom profilers

Some sub-bottom profiling systems emit sound pulses across a band of frequencies rather than a fixed frequency. These "chirp" systems produce a strong outgoing signal that returns high-resolution information on subsurface sediments. The power of the outgoing signal, oceanographic and environmental conditions, sediment composition, and seafloor morphology all influence how well sound energy penetrates the seafloor. Coarse sediments, like sand, are more difficult for sound to penetrate than finer sediments like silt and clay.

Data from sub-bottom profilers are collected on shipboard computers, and hydrographers can generate cross-sections or vertical slices depicting sediments below the seafloor. By interpreting these cross-sections, hydrographers can identify features such as volcanic ridges, underwater landslides, ancient riverbeds, and other geological features. Sub-bottom profilers provide a detailed view of the marine geological environment when used in conjunction with other data types, such as multibeam sonar, rock samples, and sediment cores.

Unmanned surface vessels (USVs)

One of the key advantages of USVs is their ability to eliminate the need for human supervision, thereby improving overall safety and reducing costs. This is especially important for near-shore bathymetric surveys, where there may be a higher risk of accidents or hazards. By deploying USVs, surveyors can minimise the risk to personnel while still achieving accurate and efficient data collection.

USVs are also highly versatile and can be equipped with advanced technologies to meet specific survey requirements. For instance, the CEE-USV by CEE HydroSystems is a state-of-the-art survey boat that incorporates a single beam echo sounder, GNSS positioning, live video, and on-board data management. This combination of features enables real-time bathymetric survey results with high accuracy and reliability.

Additionally, USVs offer greater flexibility in terms of deployment and data transmission. For example, the Kingfisher USV by Clearpath Robotics is equipped with survey-grade sonar, an inertial measurement unit, a remote controller Wi-Fi system, and survey-grade RTK GPS. This allows the remote operator to receive survey data in real time and make instant adjustments if needed. The compact and portable design of the Kingfisher also makes it well-suited for near-shore environments, as it can be easily transported and deployed in a variety of locations.

Another advantage of USVs is their ability to extend mission durations and enhance endurance. This is particularly useful for long-duration bathymetric surveys, where traditional vessels may be limited by fuel or battery constraints. The integration of solar power systems, such as those used by the ABES platform, can significantly extend mission durations by providing a secondary power source to complement traditional batteries. This not only increases the range of USVs but also reduces the need for frequent returns to shore for recharging or refuelling.

In summary, USVs offer a range of benefits that make them well-suited for near-shore bathymetric surveying. Their ability to enhance safety, improve efficiency, and extend mission durations makes them a compelling choice for hydrographic survey operations in near-shore environments. With advancements in technology and increasing demand for unmanned systems, USVs are likely to play an even more significant role in the future of bathymetric surveying.

Topographic mapping

For instance, the engineering firm McLaren has used topographic mapping for the development of several bridge and marine projects, including Domino Park, Weehawken Pier, and the Westchester Bundled Bridges.

Additionally, the use of sUAS or drones can offer a cost-effective and flexible solution for data collection, particularly in hard-to-reach or hazardous areas. They can also be easily deployed and provide high-quality data with minimal setup requirements.

Overall, topographic mapping is a valuable tool that can enhance the accuracy, safety, and efficiency of bathymetric surveys, particularly for projects involving bridges and nearshore embankments.

Frequently asked questions