Multibeam Echosounder Measurements Research Articles

Mussel culture monitoring with semi-supervised machine learning on multibeam echosounder data using label spreading

High diversity seabed habitats, such as shellfish aggregations, play a significant role in marine ecosystem sustainability but are susceptible to bottom disturbance induced by anthropogenic activities. Regular monitoring of these habitats with effective mapping methods is therefore essential. Multibeam echosounder (MBES) has been widely used in recent decades for seabed characterization due to its non-destructive manner and extensive spatial coverage compared to traditional methods like bottom sampling. Nevertheless, bottom sampling remains essential to link ground truth with acoustic seabed classification. Using seabed samples and MBES measurements, machine learning techniques are commonly employed to model their relationships and generate classification maps of an extended seabed. However, limited ground truth data, resulting from constraints in regulations, budget, or time, may impede the development of robust machine learning models. To address this challenge, we applied a semi-supervised machine learning method to classify seabed sediments of a blue mussel (Mytilus edulis) cultivation area in the Oosterschelde, the Netherlands. We utilized nine boxcore samples to generate pseudo-labels on MBES data. These pseudo-labels enlarged the training data size, facilitated the training of three comprehensive machine learning algorithms (Gradient Boosting, Random Forest, and Support Vector Machine), and helped to classify the study site into mussel and non-mussel areas. We found the geomorphological and backscatter-related features to be complementary for mussel culture detection. Our classification results were demonstrated effective through expert knowledge of this cultivation area and brought insights for future research on natural mussel habitats.

Evaluation of Active and Passive UAV-Based Surveying Systems for Eulittoral Zone Mapping

Abstract. The eulittoral zone, which alternates between being exposed and submerged, presents a challenge for high-resolution characterization. Normally, its mapping is divided between low and high water levels, where each calls for a different type of surveying instrument. This leads to inconsistent mapping products, both in accuracy and resolution. Recently, uncrewed airborne vehicle (UAV) based photogrammetry was suggested as an available and low-cost solution. However, relying on a passive sensor, this approach requires adequate environmental conditions, while its ability to map inundated regions is limited. Alternatively, UAV-based topo-bathymetric laser scanners enable the acquisition of both submerged and exposed regions independent of lighting conditions while maintaining the acquisition flexibility. In this paper, we evaluate the applicability of such systems in the eulittoral zone. To do so, both topographic and topo-bathymetric LiDAR sensors were loaded on UAVs to map a coastal region along the river Rhein. The resulting point clouds were compared to UAV-based photogrammetric ones. Aspects such as point spacing, absolute accuracy, and vertical offsets were analysed. To provide operative recommendations, each LiDAR scan was acquired at different flying altitudes, while the photogrammetric point clouds were georeferenced based on different exterior information configurations. To assess the riverbed modelling, we compared the surface model acquired by the topo-bathymetric LiDAR sensor to multibeam echosounder measurements. Our analysis shows that the accuracies of the LiDAR point clouds are hardly affected by flying altitude. The derived riverbed elevation, on the other hand, shows a bias which is linearly related to water depth.

Towards better differentiation of archaeological objects based on geomorphometric features of a digital elevation model, the case of the Old Oder Canal

AbstractLimited visibility in the underwater environment often restricts opportunities for archaeological prospection. Especially in reservoirs with a high content of suspended solids, methods based on acoustics prove to be extremely useful. This study represents the first high‐resolution acoustic mapping and archaeological prospections of the Old Oder Canal, which has extremely poor visibility. The study site is located near the town of Krosno Odrzańskie in Poland. The town is one of the country's most significant river crossings and settlements of mediaeval origin (including its wooden bridges). The following research objectives were identified: (1) exploration of the Old Oder Canal, including underwater acoustics and archaeological prospections; (2) analysis and interpretation of the study area based on acquired datasets; and (3) evaluation of secondary features of the river bathymetry for identification of archaeological objects. Possible locations of archaeological objects were determined based on analysis and interpretation of multibeam echosounder measurements of the riverbed. Fieldwork allowed structural elements of mediaeval bridges to be found and dendrochronological sampling performed. Feature selection analyses allowed the determination and evaluation of geomorphometric attributes, combining the characteristics of the discovered objects and diagnostics in order better to differentiate archaeological remains. Proposed secondary features may facilitate archaeological explorations in difficult environments.

Spatial and Temporal Changes of Tidal Inlet Using Object-Based Image Analysis of Multibeam Echosounder Measurements: A Case from the Lagoon of Venice, Italy

Scientific exploration of seabed substrata has significantly progressed in the last few years. Hydroacoustic methods of seafloor investigation, including multibeam echosounder measurements, allow us to map large areas of the seabed with unprecedented precision. Through time-series of hydroacoustic measurements, it was possible to determine areas with distinct characteristics in the inlets of the Lagoon of Venice, Italy. Their temporal variability was investigated. Monitoring the changes was particularly relevant, considering the presence at the channel inlets of mobile barriers of the Experimental Electromechanical Module (MoSE) project installed to protect the historical city of Venice from flooding. The detection of temporal and spatial changes was performed by comparing seafloor maps created using object-based image analysis and supervised classifiers. The analysis included extraction of 25 multibeam echosounder bathymetry and backscatter features. Their importance was estimated using an objective approach with two feature selection methods. Moreover, the study investigated how the accuracy of classification could be affected by the scale of object-based segmentation. The application of the classification method at the proper scale allowed us to observe habitat changes in the tidal inlet of the Venice Lagoon, showing that the sediment substrates located in the Chioggia inlet were subjected to very dynamic changes. In general, during the study period, the area was enriched in mixed and muddy sediments and was depleted in sandy deposits. This study presents a unique methodological approach to predictive seabed sediment composition mapping and change detection in a very shallow marine environment. A consistent, repeatable, logical site-specific workflow was designed, whose main assumptions could be applied to other seabed mapping case studies in both shallow and deep marine environments, all over the world.

Monitoring underwater nourishments using multibeam bathymetric and backscatter time series

Natural and man-induced coastal erosion endanger life and environment in coastal areas worldwide. For sedimentary barrier coasts, beach and underwater nourishments are an efficient coastal protection strategy. To optimize nourishments and to understand their impact on the marine environment, monitoring strategies are required. In this study, we investigate the potential of multibeam echosounder (MBES) data, providing both bathymetry and backscatter (BS), for monitoring the evolution of the nourished sediment and morphodynamics over time. A time series of seven MBES measurements, as well as two sets of box cores, vibrocores and seismic data were acquired of a channel-side nourishment near the Wadden Sea island Ameland (The Netherlands), between April 2017 and May 2019. In general, a high confidence of the acoustic reliability of the BS time series measurements is demonstrated. The unsupervised Bayesian classification method, supported by ground-truthing, is employed to produce a time series of sediment maps, revealing sediments ranging from sandy mud to sand with varying amounts of shell fragments. Based on the sediment maps, the nourished sediment could be distinguished from the natural sediment. Within one year, the shell-rich pre-nourishment seabed is recreated by washing out finer sediments, which are deposited towards the main tidal channel. Using the seismic data and vibrocores, the shell-rich pre-nourishment seabed could be identified in the subsurface after being buried by the nourishments, supporting the general findings. Furthermore, a rapid development of steep bedforms with increasing sediment sorting is observed in parts of the nourished areas. This study shows that high-resolution sediment maps obtained from a time series of MBES BS together with bathymetry reveal morphodynamic and sedimentary processes of nourishment evolution and can advance underwater nourishment strategies.

Correcting Multibeam Echosounder Bathymetric Measurements for Errors Induced by Inaccurate Water Column Sound Speeds

In this contribution a method for correcting bathymetric measurements affected by inaccurate water column sound speed profiles (SSPs) is presented. The method exploits the redundancy in the multibeam echosounder measurements obtained from the overlap of adjacent swaths by minimizing the difference between depths along overlapping swaths. Two optimization methods are used, i.e., Differential Evolution (DE) and Gauss-Newton (GN). While DE inverts for the sound speed by minimizing the depth variation, GN inverts for both bathymetry and sound speed by minimizing the squared sum of the differences between the modeled and measured travel times. The inversion method assumes a constant SSP in the water column. Applying the method to a salt wedge survey area with large variations in the water column sound speed indicates a good agreement between the original depth measurements and those derived after the inversion with the mean and standard deviation of the depth differences equaling 0.009m and 0.024m, respectively. This indicates that even with a simple parametrization of the sound speed in the water column, the correct bathymetry can be derived from the inversion. The SSP inversion method is also applied to an area with existing refraction artefacts. It corrects the bathymetry and reduces the mean and standard deviation of the depth standard deviation by a factor of around 2.75 compared to the case where the measured SSPs were used. Furthermore, the SSP inversion method neither manipulates the existing morphology nor introduces artificial bathymetric features in the areas where such refraction artefacts are not present. Considering constant SSPs, both DE and GN give almost identical results with GN being faster. However, GN is less flexible with regards to varying sound speed parameterizations.

Nearshore Benthic Habitat Mapping Based on Multi-Frequency, Multibeam Echosounder Data Using a Combined Object-Based Approach: A Case Study from the Rowy Site in the Southern Baltic Sea

Recently, the rapid development of the seabed mapping industry has allowed researchers to collect hydroacoustic data in shallow, nearshore environments. Progress in marine habitat mapping has also helped to distinguish the seafloor areas of varied acoustic properties. As a result of these new developments, we have collected a multi-frequency, multibeam echosounder dataset from the valuable nearshore environment of the southern Baltic Sea using two frequencies: 150 kHz and 400 kHz. Despite its small size, the Rowy area is characterized by diverse habitat conditions and the presence of red algae, unique on the Polish coast of the Baltic Sea. This study focused on the utilization of multibeam bathymetry and multi-frequency backscatter data to create reliable maps of the seafloor. Our approach consisted of the extraction of 70 secondary features of bathymetric and backscatter data, including statistic and textural attributes of different scales. Based on ground-truth samples, we have identified six habitat classes and selected the most relevant features of the bathymetric and backscatter data. Additionally, five types of image processing pixel-based and object-based classifiers were tested. We also evaluated the performance of algorithms using an accuracy assessment based on the validation subset of the ground-truth samples. Our best results reached 93% overall accuracy and a kappa coefficient of 0.90, confirming that nearshore seabed habitats can be accurately distinguished based on multi-frequency, multibeam echosounder measurements. Our predictive habitat mapping of shallow euphotic zones creates a new scientific perspective and provides relevant data for the management of natural resources. Object-based approaches previously used in various environments and areas suggest that methodology presented in this study may be scalable.

Spatial Structure of the Antarctic Water Flow in the Vema Fracture Zone of the Mid-Atlantic Ridge

We study the Antarctic Bottom Water (AABW) flow in the Vema Fracture Zone of the Mid-Atlantic Ridge using the high-resolution Institute of Numerical Mathematics Ocean Model (INMOM) and data from field measurements. The key feature of this numerical modeling is high horizontal and vertical resolution in the bottom layer for the simulation of the flow in the narrow deepwater fracture, as well as the use of high-quality topography based on multibeam echo sounder measurements. Direct CTD and LADCP measurements performed onboard the R/V Akademik Sergey Vavilov in 2006 and 2014–2016 were used to verify the model. In this work, we analyze both the thermohaline structure of the bottom layer in the Vema Fracture Zone and kinematics of the flow over its entire length.

Seafloor mapping based on multibeam echosounder bathymetry and backscatter data using Object-Based Image Analysis: a case study from the Rewal site, the Southern Baltic

Abstract Seafloor mapping is a fast developing multidisciplinary branch of oceanology that combines geophysics, geostatistics, sedimentology and ecology. One of its objectives is to isolate distinct seabed features in a repeatable, fast and objective way, taking into consideration multibeam echosounder (MBES) bathymetry and backscatter data. A large-scale acoustic survey was conducted by the Maritime Institute in Gdańsk in 2010 using Reson 8125 MBES. The dataset covered over 20 km2 of a shallow seabed area (depth of up to 22 m) in the Polish Exclusive Economic Zone within the Southern Baltic. Determination of sediments was possible based on ground-truth grab samples acquired during the MBES survey. Four classes of sediments were recognized as muddy sand, very fine sand, fine sand and clay. The backscatter mosaic created using the Angular Variable Gain (AVG) empirical method was the primary contribution to the image processing method used in this study. The use of the Object-Based Image Analysis (OBIA) and the Classification and Regression Trees (CART) classifier makes it possible to isolate the backscatter image with 87.5% overall and 81.0% Kappa accuracy. The obtained results confirm the possibility of creating reliable maps of the seafloor based on MBES measurements. Once developed, the OBIA workflow can be applied to other spatial and temporal scenes.

Malaysia Airlines flight MH370 search data reveal geomorphology and seafloor processes in the remote southeast Indian Ocean

A high-resolution multibeam echosounder (MBES) dataset covering over 279,000km2 was acquired in the southeastern Indian Ocean to assist the search for Malaysia Airlines Flight 370 (MH370) that disappeared on 8 March 2014. The data provided an essential geospatial framework for the search and is the first large-scale coverage of MBES data in this region. Here we report on geomorphic analyses of the new MBES data, including a comparison with the Global Seafloor Geomorphic Features Map (GSFM) that is based on coarser resolution satellite altimetry data, and the insights the new data provide into geological processes that have formed and are currently shaping this remote deepsea area. Our comparison between the new MBES bathymetric model and the latest global topographic/bathymetric model (SRTM15_plus) reveals that 62% of the satellite-derived data points for the study area are comparable with MBES measurements within the estimated vertical uncertainty of the SRTM15_plus model (±100m). However, >38% of the SRTM15_plus depth estimates disagree with the MBES data by >100m, in places by up to 1900m. The new MBES data show that abyssal plains and basins in the study area are significantly more rugged than their representation in the GSFM, with a 20% increase in the extent of hills and mountains. The new model also reveals four times more seamounts than presented in the GSFM, suggesting more of these features than previously estimated for the broader region. This is important considering the ecological significance of high-relief structures on the seabed, such as hosting high levels of biodiversity. Analyses of the new data also enabled sea knolls, fans, valleys, canyons, troughs, and holes to be identified, doubling the number of discrete features mapped.Importantly, mapping the study area using MBES data improves our understanding of the geological evolution of the region and reveals a range of modern sedimentary processes. For example, a large series of ridges extending over approximately 20% of the mapped area, in places capped by sea knolls, highlight the preserved seafloor spreading fabric and provide valuable insights into Southeast Indian Ridge seafloor spreading processes, especially volcanism. Rifting is also recorded along the Broken Ridge – Diamantina Escarpment, with rift blocks and well-bedded sedimentary bedrock outcrops discernible down to 2400m water depth. Modern ocean floor sedimentary processes are documented by sediment mass transport features, especially along the northern margin of Broken Ridge, and in pockmarks (the finest-scale features mapped), which are numerous south of Diamantina Trench and appear to record gas and/or fluid discharge from underlying marine sediments. The new MBES data highlight the complexity of the search area and serve to demonstrate how little we know about the vast areas of the ocean that have not been mapped with MBES. The availability of high-resolution and accurate maps of the ocean floor can clearly provide new insights into the Earth's geological evolution, modern ocean floor processes, and the location of sites that are likely to have relatively high biodiversity.

Characterization of Seafloor Roughness to Support Modeling of Midfrequency Reverberation

A seafloor laser scanner was deployed in the Gulf of Mexico during the 2013 Target and Reverberation Experiment (TREX13). This system collected digital elevation maps at 14 locations along the main reverberation track, and these measurements provide roughness power spectra for modeling seafloor acoustic scattering. The spectra were divided into two regimes according to the mid and high-frequency acoustic measurements made during the experiment. For the wave numbers corresponding to the midfrequency regime (2–4 kHz), the spectra could be approximated using the mean spectral exponent derived from the all of the spectra. With this spectral exponent, the best fit spectral strengths were found to be negatively correlated to the backscatter levels measured at 400 kHz using a multibeam echosounder (MBES). While the scattering mechanisms at 400 kHz are not influenced by the roughness at these low wave numbers, this correlation may be indirectly related to the bioturbation and the spatial variation of the shell content. A more pronounced correlation was found for the high wave numbers, where again a single spectral exponent could be used to a good approximation. In this case, the spectral strengths were also linearly related to the MBES backscattering level but with a positive correlation. For these wave numbers, the roughness is largely influenced by the shell content, which is also the dominant scattering mechanism at 400 kHz. The correlations between the roughness and the MBES measurements provide a means to approximate the seafloor roughness parameters in both wave number regimes throughout the experiment site. For the low wave number spectrum, an alternative approach is also proposed, which uses the spectral parameters for the mean spectrum to approximate the roughness throughout the TREX13 site.

Change Detection Of Seafloor Topography By Modeling Multitemporal Multibeam Echosounder Measurements

Abstract. The term "topography" implies the study of numerous landforms that exist on or below the Earth and a detailed knowledge of topography is required to understand the most Earth processes. In the oceans, sea floor topography refers the geographic features of the sea floor including the configuration of a surface and the position of its natural and man-made features; and detailed nautical charts are fundamental for many sciences such as physical oceanography, biology and marine geology. The hydrographic offices, which use the Multi Beam Echo sounder (MBE) system for the establishment of nautical charts, have their own set of accuracy standards for hydrographic surveys, which generally comply with the standards defined by the International Hydrographic Organization. MBE systems include multiple measurement systems such as sonar head, positioning system, motion sensor that work in a synchronized manner. Before the measurements, the "Patch Test" is required to eliminate the systematic errors due to instrumental synchronization and installation. In this test, signal delay test (latency), Y-axis rotation (roll), X-axis rotation (pitch), Z-axis rotation (yaw) errors are calculated. Besides, the effects of the sound velocity measurement through water column and the sea level changes need to be taken into consideration especially in the multi-temporal data analysis and 3D modeling. In this paper, the seafloor of the Anamur -TRNC Drinking Water Pipeline route in the "Northern Cyprus Water Project" is selected as a study area. This project, a unique in the world, is an international water diversion project designed to supply water for drinking and irrigation from southern Turkey to Northern Cyprus via pipeline under Mediterranean Sea. Multi temporal multi beam echo sounder measurements are used in the change analysis and surface modeling and the efficiency of this system is outlined together with its limitations.

High-frequency multibeam echo-sounder measurements of seafloor backscatter in shallow water: Part 2 – Mosaic production, analysis and classification

Backscatter measurements collected with high-frequency multibeam echo-sounders (MBES) have been used to map the spatial distribution of seafloor types and properties in shallow waters. The methods for collection and processing of MBES backscatter data were considered in Part 1 – Data acquisition and processing (Parnum and Gavrilov, 2011), published alongside this paper. The aim of Part 2 is to describe the main methods used to produce backscatter maps of the sea floor free from angular artefacts, and to classify seafloor types based on backscatter and bathymetry analysis. These methods are illustrated using a dataset collected with a Reson SeaBat 8125 MBES in Esperance Bay in Western Australia, although the methods and errors discussed are relevant to other high-frequency shallow-water MBES.

High-frequency multibeam echo-sounder measurements of seafloor backscatter in shallow water: Part 1 – Data acquisition and processing

Abstract Methods for processing multibeam echo-sounder (MBES) bathymetry data are well established, how-ever, there is no universal and established approach to processing and interpretation of MBES backscatter data. The aim of this paper is to (1) detail the different backscatter logging methods implemented in modern high-frequency MBES systems used primarily in shallow water; (2) describe and suggest appropriate methods of data processing; and (3) discuss likely errors and artefacts expected from MBES backscatter processing. These issues are illustrated using data collected with a Reson SeaBat MBES in coastal shelf areas of Australia, although the methods and errors discussed are relevant to other high-frequency shallow-water MBES systems. Production of backscatter mosaics and analysis of backscatter for seafloor classification using MBES data are considered in Part 2 – Mosaic production, analysis and classification (Parnum and Gavrilov, 2011), published alongside this paper.

Consecutive acoustic observations of an Atlantic herring school in the Northwest Atlantic

Abstract Weber, T. C., Peña, H., and Jech, J. M. 2009. Consecutive acoustic observations of an Atlantic herring school in the Northwest Atlantic. – ICES Journal of Marine Science, 66: 1270–1277. Several successive images of the same school of Atlantic herring (Clupea harengus) were collected over the course of ∼1 h just north of Georges Bank in the Northwest Atlantic. Although the fish may not have been in their natural, undisturbed state, we observed what appeared to be the fish school fragmenting and dispersing, using a split-beam and a multibeam echosounder. Calibrated, 38 kHz, split-beam echosounder (Simrad EK60) and trawl-catch data provided accurate measures of the fish density beneath the vessel. Uncalibrated, 400 kHz, multibeam-echosounder (Reson 7125) data provided synoptic observations of the fish school including estimates of the school volume, morphology, and behaviour. Observations of the angular dependence in the multibeam-echosounder measurements of backscatter from fish allow investigation of the efficacy of extrapolating fish-school densities measured by the split-beam echosounder to the entire school.

A comparison between modeled and measured high frequency bottom backscattering

Compared to taking many sediment grabs, classification techniques based on acoustic remote sensing allow for significant cost reductions, providing the required information at much denser spatial scale. An attractive approach hereto is to use the multibeam-echosounder (MBES) which measures sediment backscatter strengths as a function of angle. By maximizing the match between this backscatter strength and model results, sediment properties can be derived. The backscatter model used here accounts for both surface roughness and volume scattering. MBES measurements (300 kHz) were taken in the Cleaver-Bank area (North Sea), characterized by a wide variety of sediment types for which a large number of grabs is collected. For the majority of the sediment types occurring, well-defined single-sediment-type areas can be discerned. Based on MBES measurements in these areas and corresponding geotechnical sediment parameters provided by grab grain size analysis, calibration curves are derived, representing the mismatch between modeled and measured backscatter strengths. These curves are found to be similar for all sediment types considered. By applying these correction curves to the MBES measurements, they show good agreement with the model for all observed sediment types. It is shown that after this correction the model can be used for sediment classification purposes.

Correcting bathymetry measurements for water sound speed effects using inversion theory

We present a method to accurately estimate the bathymetry and water sound speed in shallow waters using overlapping swaths obtained from a Multi-Beam Echo Sounder (MBES). The method is designed to correct the errors in the bathymetry manifested mainly in the outer beams of the MBES. The errors are caused by deviations in sound speed, which occur, for example, in estuaries where fresh river water mixes with seawater. Simulations show that we are able to simultaneously estimate the mean sound speed and the bathymetry. This is accomplished by minimizing the differences between the MBES measurements in the overlap region using a Levenberg-Marquardt optimization routine. We also present an application of the method to real data obtained at the fairway to the harbour of Rotterdam, The Netherlands. Analysis shows that the inverted sound speed agrees well with the measured sound speed at the transducer. However, the uncertainties are larger when the track-to-track distance is ̃2 times the depth. Therefore, we also discuss the optimal distance between sailed tracks. The method appears to be a promising tool for the accurate mapping of sea floors in shallow-water areas with a complex sound-speed profile.

Estimation of macrophytes using single-beam and multibeam echosounding for environmental monitoring of arctic fjords (Kongsfjord, West Svalbard Island)

This paper presents results of a study on the spatial distribution and biomass of macrophytobentos in a fjord of Arctic Svalbard. Kongsfjord represents a periglacial environment with intense morphodynamic processes and rapidly progressing changes in the biotic environment, making it one of the most promising areas to research climate impact on ecosystems. The main objective was to provide an acoustic tool for the evaluation of benthic habitats. The 2007 field survey included systematic, co-registered, single-beam and multibeam echosounder measurements. Acoustic observations were verified by biological samplings and observations for the classification algorithm development and verification. Analyses of acoustic signals scattered on bottoms covered by algae indicate the good quality of the data recorded, providing a map of phytobenthos distribution and biomass estimation in Kongsfjord. The algorithms designed and tested for processing single- and multibeam data allow extracting the morphological forms of the bottom and determine the areas covered by algae. This survey was the first instance where a multibeam imager was used to map macrophytes in an Arctic environment, in a wide variety of depths and ranges. Comparing the two types of echosounders was a great opportunity to understand the visualisation of macrophytes by multibeam systems and their identification.

Using the MBES for classification of riverbed sediments

For keeping the Dutch rivers suitable for commercial activities measures are required. For example, the bottom of the river Waal, connecting Rotterdam with German industrial areas, is subsiding. Since the subsidence varies along the river, dangerous shoals occur. Sediment suppletions are planned to counteract the subsidence. Appropriate suppletion material is expected to keep the bottom more stable. To monitor the suppletion effectiveness, multibeam‐echosounder (MBES) measurements are planned, allowing for simultaneous estimation of bathymetry and sediment composition. For the latter, we apply a method employing the MBES backscatter data. It estimates the number of sediment types present in the survey area and discriminates between them by applying the Bayes decision rule for multiple hypotheses, implicitly accounting for the backscatter strength ping‐to‐ping variability. The method's applicability was demonstrated in a well‐surveyed test area (North Sea). In 2007, MBES measurements were acquired at the Waal, accompanied with extensive sediment grabbing. Contrary to the test area, water depths are very shallow and significant bottom slopes exist, requiring corrections. The lower water depths correspond to smaller beam‐footprints, resulting in a higher ping‐to‐ping variability. Consequently the discriminating power between sediments will decrease. The performance of the classification method for this river environment is assessed.

Survey Boat Attitude Determination with GPS/IMU Systems

Any vehicle such as a vessel or aeroplane has three attitude parameters. These are most commonly defined as pitch, roll and heading from true north. In hydrographic surveying, determination of these parameters is essential for the correction of multi-beam echo sounder measurements. In the study on which this paper is based, two of the three parameters, the pitch and roll angles, were measured with an inexpensive IMU (Inertial Measurement Unit) device. The third was calculated from GPS carrier phase measurements that were collected from two antennas on the boat. The GPS antennas depart from the vertical when they are subject to pitch and roll effects. In this case, the coordinates derived from GPS will be erroneous. Thus, if heading is to be calculated accurately, the horizontal coordinates have to be corrected for pitch and roll. This paper defines an algorithm for the purpose that enables pitch and roll angles to be measured with an accuracy of about 0·003 arcdeg and headings computed with an accuracy of about 0·010 arcdeg.