hydrographic survey equipment

Hydrographic Survey Equipment

A Hydrographic Survey Equipment list is vast. The Industry covers everything from Shallow water single beam echo sounders to full ocean depth multibeam echosounders. While Bathylogger focuses on the bl200 entry level sonar echosounders and BathyCat USV (Unmanned Surface Vessels) we call RC Survey Boats, below is a list of some of the most popular types of Bathymetric / Hydrographic survey Gear used in the industry today. In areas where detailed bathymetry is required, a precise echo sounder may be used for the work of hydrography. There are many considerations when evaluating such a system, not limited to the vertical accuracy, resolution, acoustic beamwidth of the transmit/receive beam and the acoustic frequency of the transducer.
The majority of hydrographic echosounders are single or dual frequency, meaning that a low frequency pulse (typically around 24 kHz) can be transmitted at the same time as a high frequency pulse (typically around 200 kHz). As the two frequencies are discrete, the two return signals do not typically interfere with each other.

Most hydrographic operations use a 200 kHz transducer, which is suitable for inshore work up to 100 metres in depth. Deeper water requires a lower frequency transducer as the acoustic signal of lower frequencies is less susceptible to attenuation in the water column. Commonly used frequencies for deep water sounding are 33 kHz and 24 kHz. The beamwidth of the transducer is also a consideration for the hydrographer, as to obtain the best resolution of the data gathered a narrow beamwidth is preferable. This is especially important when sounding in deep water, as the resulting footprint of the acoustic pulse can be very large once it reaches a distant sea floor.

In addition to the single beam echo sounder, there are echo sounders that are capable of receiving many return “pings”. These systems are detailed further in the section called multibeam echosounder. Echo sounders are used in laboratory applications to monitor sediment transport, scour and erosion processes in scale models (hydraulic models, flumes etc.). These can also be used to create plots of 3D contours.

The Single beam echo sounder type, like the bl200 portable sonar kit is the most widely used sonar in the world. Most jobs like dredging, simple scouring, river cross sections etc don’t require image grade sonar like a multibeam echosounder. The newest single beams echo sounders are better than ever. More accurate, lighter and easier to use.

The Mulitibeam Echosounder has come a long way since the early SeaBat. They too are smaller and more powerful with over 500 beams pinging at 50 Hrz in a wide SWATH or FAN array in the top of the line Models like the RESON and R2 Sonic. The smaller lower cost units like the PICO are finally getting very good shallow water imagery. So the Multibeam is a bathymetry sonar which means every sounding / point is an actual accurate recorded depth. As you can imagine 500 beams at 50 times per second your going to have one hell of a point cloud and very fine detail of the Seafloor. These are often compared to Lidar of the Sea. Multibeam sonar requires advances setup procedure and a minimum of a few weeks of Training. On acquiring and post processing data. see the multibeam DAM image. Multibeam echo sounders (MBES), like other sonar systems, transmit sound energy and analyze the return signal (echo) that has bounced off the seafloor or other objects. Multibeam sonars emit sound waves from directly beneath a ship’s hull to produce fan-shaped coverage of the seafloor. These systems measure and record the time for the acoustic signal to travel from the transmitter (transducer) to the seafloor (or object) and back to the receiver. Multibeam sonars produce a “swath” of soundings (i.e., depths) to ensure full coverage of an area. The coverage area on the seafloor is dependent on the depth of the water, typically two to four times the water depth.

Many MBES systems are capable of recording acoustic backscatter data. Multibeam backscatter is intensity data that can be processed to create low resolution imagery. Backscatter is co-registered with the bathymetry data and is often used to assist with bathymetric data interpretation and post-processing. NOAA hydrographic survey units use mutlibeam echo sounder systems to acquire full- and partial- bottom bathymetric coverage throughout a survey area, to determine least depths over critical items such as wrecks, obstructions, and dangers-to-navigation, and for general object detection. NOAA field units use various models of swath-type multibeam systems both hull and pole mounted for hydrographic survey operations. Mutlibeam echo sounder systems employed must meet specifications outlined in the NOS Hydrographic Surveys Specifications and Deliverables.

Sidescan sonar Side-scan sonar may be used to conduct surveys for marine archaeology; in conjunction with seafloor samples it is able to provide an understanding of the differences in material and texture type of the seabed. Side-scan sonar imagery is also a commonly used tool to detect debris items and other obstructions on the seafloor that may be hazardous to shipping or to seafloor installations by the oil and gas industry. In addition, the status of pipelines and cables on the seafloor can be investigated using side-scan sonar. Side-scan data are frequently acquired along with bathymetric soundings and sub-bottom profiler data, thus providing a glimpse of the shallow structure of the seabed. Side-scan sonar is also used for fisheries research, dredging operations and environmental studies. It also has military applications including mine detection. Side-scan uses a sonar device that emits conical or fan-shaped pulses down toward the seafloor across a wide angle perpendicular to the path of the sensor through the water, which may be towed from a surface vessel or submarine, or mounted on the ship’s hull. The intensity of the acoustic reflections from the seafloor of this fan-shaped beam is recorded in a series of cross-track slices. When stitched together along the direction of motion, these slices form an image of the sea bottom within the swath (coverage width) of the beam. The sound frequencies used in side-scan sonar usually range from 100 to 500 kHz; higher frequencies yield better resolution but less range.

Side scan sonar is a specialized sonar system for searching and detecting objects on the seafloor. Like other sonars, a side scan transmits sound energy and analyzes the return signal (echo) that bounced off the seafloor or other objects. Side scan sonar typically consists of three basic components: a towfish, a transmission cable and the topside processing unit. In a side scan the transmitted energy is formed into the shape of a fan that sweeps the seafloor from directly under the towfish to either side, typically to a distance of 100 meters.

The strength of the return echo is continuously recorded creating a “picture” of the ocean bottom. For example, objects or features that protrude from the seafloor create a strong return (creating a light area) and shadows from these objects create little or no return signal (creating a dark area). While the shape of the seafloor and objects on it can be well-depicted, most side scan systems cannot provide any depth information. NOAA hydrographic survey units use side scan sonar systems for both object detection and object recognition. Side scan sonar is typically used in conjunction with a single beam or multibeam sonar system to meet full bottom coverage specifications for Coast Survey surveys. NOAA field units use various models of side scan sonar in both hull mounted and towed configurations for hydrographic survey operations. Any side scan sonar system employed must meet the specifications outlined in the NOS Hydrographic Surveys Specifications and Deliverables

Unmanned surface vehicles (USV) or autonomous surface vehicles (ASV) are vehicles that operate on the surface of the water (watercraft) without a crew. The Bathylogger BathyCat RC / Autonomous Suvey Boat is a USV, used primary to conduct single beam echosounder surveys, but can be fitted for many other missions.

USVs are valuable in oceanography, as they are more capable than moored or drifting weather buoys, but far cheaper than the equivalent weather ships and research vessels,[1] and more flexible than commercial-ship contributions. Wave gliders, in particular, harness wave energy for primary propulsion[2] and, with solar cells to power their electronics, have months of marine persistence[3] for both academic [4][5] and naval applications.[6][7]

Powered USVs are popular for use in hydrographic survey. Using a small USV in parallel to traditional survey vessels as a ‘force-multiplier’ can double survey coverage and reduce time on-site. This method was used for a survey carried out in the Bering Sea, off Alaska; the ASV Global ‘C-Worker 5′ autonomous surface vehicle (ASV) collected 2,275 nautical miles of survey, 44% of the project total. This was a first for the survey industry and resulted in a saving of 25 days at sea.[8]

Military applications for USVs include powered seaborne targets and minehunting. Operational USVs with offensive capability include the Israeli Protector USV.[9] In the future, many unmanned cargo ships are expected to cross the water.

A remotely operated underwater vehicle (technically ROUV but commonly just ROV) is a tethered underwater mobile device. This meaning is different from remote control vehicles operating on land or in the air. ROVs are unoccupied, usually highly maneuverable, and operated by a crew either aboard a vessel/floating platform or on proximate land. They are common in deep water industries such as offshore hydrocarbon extraction. They are linked to a host ship by a neutrally buoyant tether or, often when working in rough conditions or in deeper water, a load-carrying umbilical cable is used along with a tether management system (TMS). The TMS is either a garage-like device which contains the ROV during lowering through the splash zone or, on larger work-class ROVs, a separate assembly which sits on top of the ROV. The purpose of the TMS is to lengthen and shorten the tether so the effect of cable drag where there are underwater currents is minimized. The umbilical cable is an armored cable that contains a group of electrical conductors and fiber optics that carry electric power, video, and data signals between the operator and the TMS. Where used, the TMS then relays the signals and power for the ROV down the tether cable. Once at the ROV, the electric power is distributed between the components of the ROV. However, in high-power applications, most of the electric power drives a high-power electric motor which drives a hydraulic pump. The pump is then used for propulsion and to power equipment such as torque tools and manipulator arms where electric motors would be too difficult to implement subsea. Most ROVs are equipped with at least a video camera and lights. Additional equipment is commonly added to expand the vehicle’s capabilities. These may include sonars, magnetometers, a still camera, a manipulator or cutting arm, water samplers, and instruments that measure water clarity, water temperature, water density, sound velocity, light penetration, and temperature.[1] Also optical-stereo cameras have been mounted on ROVs in order to improve the pilots’ perception of the underwater scenario.[2]

A sound velocity profiler is a device that is used for measuring the speed of sound, specifically in the water column, for oceanographic and hydrographic research purposes. There are two common methods to obtaining sound velocity in water using the probe method.

Firstly, the main three variables that affect sound velocity may be measured using a Conductivity, Temperature & Depth Probe (CTD Probe). This instrument can determine the salinity, temperature and pressure variables, and then calculate the sound velocity of the water using one of the many formulae available.[2] Secondly, the speed of sound may be directly measured using a small acoustic transducer and a reflecting surface, mounted at a known distance from the acoustic center of the transducer. If the distance from the transducer to the reflector is known, and the time taken from the transmit to the receive pulse is known, then the speed of sound in water can be calculated. Transducers used in Lidar (light detection and ranging) technology measures elevation or depth by analyzing the reflection of pulses of laser light off an object. Lidar survey systems are typically aircraft mounted and provide seamless, contiguous coverage between land and sea.

Bathymetric lidar is used to determine water depth by measuring the time delay between the transmission of a pulse and its return signal. Systems use laser pulses received at two frequencies: a lower frequency infrared pulse is reflected off the sea surface, while a higher frequency green laser penetrates through the water column and reflects off the bottom. Analyses of these two distinct pulses are used to establish water depths and shoreline elevations. With good water clarity, these systems can reach depths of 50 meters. Bathymetric lidar is also used to acquire data in areas with complex and rugged shorelines where surface vessels cannot operate efficiently or safely because of rocks, kelp or breaking surf. Some examples of these areas include Alaska, the North Atlantic Coast and the Caribbean.

The Global Positioning System (GPS) is the basis for nearly all position measurement in NOAA hydrography. In addition to providing location information for survey ships and launches, accurate positioning is required for several other elements of coastal hydrography.

Fixed Aids to Navigation (ATONs) are occasionally found to be out of their published positions. The location of these ATONS must be accurately measured for charting. NOAA utilizes static GPS survey methods to position critical Aids to sub-meter level accuracy.

NOAA utilizes USCG-maintained differential GPS beacons to improve the accuracy of position measurements. However, in some remote areas far from permanent differential beacons or where local topography blocks the beacon’s signal, NOAA establishes its own temporary DGPS station to broadcast correctors to the survey launches.

Nearshore features such as pilings and piers must be portrayed accurately on nautical charts. While remote sensing methods such as aerial photogrammetry are usually the most efficient means of positioning these items, Rainier often locates new features which do not appear in the latest remote sensing data. In these cases, hydrographers use portable backpack DGPS receivers to define the position, extents, and attributes of these items.

NOAA is also working to establish the relationship between local water level datums such as Mean Lower Low Water, and Ellipsoidal Height produced by GPS. To assist in this effort, NOAA uses dual frequency static GPS methods to measure the ellipsoidal height of water level (tide) stations to centimeter level accuracy.

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