Laser Ranging Bathymetry Using a Photon-Number-Resolving Detector (original) (raw)

Shallow Bathymetric Mapping via Multistop Single Photoelectron Sensitivity Laser Ranging

IEEE Transactions on Geoscience and Remote Sensing, 2000

We discuss the optimization of components in a single-wavelength airborne laser bathymeter that is intended for a low-power unmanned aerial vehicle platform. The theoretical minimum energy requirement to detect the submerged sea floor in shallow (< 5 m) water using a low signal-to-noise ratio (LSNR) detection methodology is calculated. Results are presented from tests of a prototype light detection and ranging (LiDAR) instrument that was developed by the University of Florida, Gainesville. A green wavelength (532 nm), 100-beamlet, low-energy (35-nJ/beamlet), short-pulse (480 ps) laser ranging system was operated from a low-altitude (500-m) aircraft, with a multichannel sensor that is capable of single photoelectron sensitivity and multiple stops. Data that were collected during tests display vertical structure in shallow-water areas based on fixed threshold crossings at a single-photon sensitivity level. A major concern for the binary detection strategy is the reliable identification and removal of noise events. Potential causes of ranging errors related to photomultiplier tube afterpulsing, impedance mismatching, and gain block overdrive are described. Data collection/processing solutions based on local density estimation are explored. Previous studies on LSNR performance metrics showed that short (15-cm) dead time could be expected in the case of multiple scattering objects, indicating the possibility of seamless topographic/bathymetric mapping with minimal discontinuity at the waterline. LiDAR depth estimates from airborne profiles are compared to on-site measurements, and near-shore submerged feature identification is presented.

ACOUSTIC AND LASER BATHYMETRY SYSTEMS

This paper discusses acoustic and lasers based bathymetric systems in terms of their applicability in diverse circumstances. The advantages and disadvantages of each method are compared in terms of capabilities, cost and accuracy. None of these systems can provide full bottom coverage in all circumstances but could be supplementary to each other. Information has been presented in support of this conclusion. Acoustic bathymetry is suitable at deeper waters whereas laser bathymetry may be used in shallow clear coastal waters. A hybrid option has been suggested with the mix of these systems for higher survey efficiency and lesser costs. This paper is of interest to persons involved in ocean acoustics study and survey projects planners as well as to the developers of laser instruments for study ocean water and bottom properties and object detection such as wracks, boulders and other objects.

Feasibility of Laser Bathymetry for Hydrographic Surveys on the Baltic Sea

International Hydrographic Review, 2014

Airborne laser bathymetry (ALB) is considered to be a new technology for hydrographic purposes in shallow waters. However, previous tests have shown greater problems especially regarding point density and the detection of obstructions. Before introducing this technology as a regular means of surveying, it is necessary to identify its possibilities and limitations, especially in the shallow waters of the Baltic Sea. New tests have been carried out in several of the Baltic Sea’s coastal states. The first results demonstrate that the accuracy of the surveys is not a major issue for using ALB in modelling the sea floor. However, point density remains a limit in deeper waters, where small obstructions cannot to be detected.

Water Surface Reconstruction in Airborne Laser Bathymetry from Redundant Bed Observations

ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences, 2017

In airborne laser bathymetry knowledge of exact water level heights is a precondition for applying run-time and refraction correction of the raw laser beam travel path in the medium water. However, due to specular reflection especially at very smooth water surfaces often no echoes from the water surface itself are recorded (drop outs). In this paper, we first discuss the feasibility of reconstructing the water surface from redundant observations of the water bottom in theory. Furthermore, we provide a first practical approach for solving this problem, suitable for static and locally planar water surfaces. It minimizes the bottom surface deviations of point clouds from individual flight strips after refraction correction. Both theoretical estimations and practical results confirm the potential of the presented method to reconstruct water level heights in dm precision. Achieving good results requires enough morphological details in the scene and that the water bottom topography is cap...

Potential of space-borne LiDAR sensors for global bathymetry in coastal and inland waters

This work aimed to prospect future space-borne LiDAR sensor capacities for global bathymetry over inland and coastal waters. The sensor performances were assessed using a methodology based on waveform simulation. A global representative simulated waveform database is first built from the Wa-LiD (Water LiDAR) waveform simulator and from distributions of water parameters assumed to be representative at the global scale. A bathymetry detection and estimation process is thus applied to each waveform to determine the bathymetric measurement probabilities in coastal waters, shallow lakes, deep lakes and rivers for a range of water depths. Finally, with a sensitivity analysis of waveforms, the accuracy and some limiting factors of the bathymetry are identified for the dominant water parameters.

Analyzing near water surface penetration in laser bathymetry-A case study at the River Pielach

Recent developments in sensor technology yielded a major progress in airborne laser bathymetry for capturing shallow water bodies. Modern topo-bathymetric small foot print laser scanners do no longer use the primary near infrared (NIR) signal (λ=1064 nm) but only emit and receive the frequency doubled green signal (λ=532 nm). For calculating correct water depths accurate knowledge of the water surface (air-water-interface) is mandatory for obtaining accurate spot positions and water depths. Due to the ability of the green signal to penetrate water the first reflections do not exactly represent the water surface but, depending on environmental parameters like turbidity, a certain penetration into the water column can be observed. This raises the question if it is even feasible to determine correct water level heights from the green laser echoes only. In this article, therefore, the near water surface penetration properties of the green laser signal are analyzed based on a test flight of the River Pielach (Austria) carried out with Riegl's VQ-820-G (532 nm) and VQ-580 (1064 nm) scanners mounted on the same airborne platform. It is shown that within the study area the mean penetration into the water column is in the range of 10-25 cm compared to the NIR signal as reference. However, as the upper hull of the green water surface echoes coincides with the NIR signal with cm-precision, it is still possible to derive water surface models from the green laser echoes only via statistical analysis of aggregated neighboring echoes and robustly keep the underestimation of the water level below 6 cm. This especially holds for still and stationary flowing water bodies.

Airborne Laser Bathymetry – detecting and recording submerged archaeological sites from the air

A new generation of airborne bathymetric laser scanners utilises short green laser pulses for high resolution hydrographic surveying in very shallow waters. The paper investigates its use for the documentation of submerged archaeological structures, introducing the concept of airborne laser bathymetry and focussing on a number of challenges this novel technology still has to face. Using this method, an archaeological pilot study on the northern Adriatic coast of Croatia has revealed sunken structures of a Roman villa. The results demonstrate the potential of this novel technique to map submerged archaeological structures over large areas in high detail in 3D, for the first time providing the possibility for systematic, large-scale archaeological investigation of this environment. The resulting maps will provide unique means for underwater heritage management.► Introducing a new method for underwater prospection of shallow waters. ► Topography of submerged archaeological sites can be mapped from the air in detail. ► Airborne laser scanner operating in the green wavelength bridge the “border” between land and water. ► Large areas can be scanned in a short time in high detail. ► Case-study of a Roman site in the northern Adriatic.

Interferometric Bathymetry-principles and utility

2013

This article presents a general way of the possibility to investigat to the bottom of rivers or sea. Measuring the response time of rising sound sonar, multibeam ecosounders systems or interferometric methods of investigation of water bottom provides precise or less precise information about topography underwater fauna or sedimentary deposits required in many human activities. Are presented here and several such projects in Romania where the bathymetry have an important role. Key-Words: signal, echosounder, bandwidth, depth, seafloor, SAS interferometry, Danube Delta

Development of Bathymetric Techniques

2015

Preparation of modern nautical charts which are very essential to marine commerce safety, location of underwater works, coastal zone management, volume of underwater excavation, volume of water in lakes, fish and mineral industries are examples of application of underwater depth. Historically, the first hydrographic method used in water depth determination was the sounding line which has then been replaced by the acoustic sounding techniques that solved the problem of deep water depth measurement. Both techniques were shipboard instruments that have the limitation of efficiency and time consuming. This is why a vast area of world water bodies are still not covered for water depth determination. Recent techniques, all classified as remote sensing techniques including airborne LIDAR, airborne and satellite sensors that work with optical or radar waves are used to determine water depth especially for vast water areas such as seas and oceans. This paper is an attempt to review technique...

Bathymetric measurements - principles and utility

2014

At this time, when marine and river navigation intensifies, it is necessary detailed knowledge of the water bottom topography of the river navigable channel and marine coastal areas. Sonar sounding systems, multibeam ecosounders systems or those using interferometry, is investigating ways water bottom topography, to identify sedimentary deposits or to achieve the necessary studies hydrotechnical constructions. The importance of bathymetric measurements is noted in several projects implemented in Romania, presented in this paper. Keywords—bandwidth, beamwidth, depth, echosounder, seafloor, signal, SAS interferometry, Danube Delta