Frequency Echo Sounder Research Articles

Determination of the relevant multibeam echosounder frequency to estimate the suspended sediment concentrations for environmental damage monitoring of mass movement

The problem of environmental damages in the river area can transform the morphology and threaten the ecosystem in it with one of the causes being natural factors such as suspended sediment. Retracing the medium form is fluid, the common instrument to determine the condition of the area is a sound wave-based instrument such as a multibeam echosounder. Considering the improvement of multibeam echosounder which can acquire areas using many frequencies at one time, noted as multi-frequency multibeam echosounder, now its application can reach various fields including environmental monitoring. Factors that can be considered in its practice include time efficiency, cost, and notably the accuracy of the data result. By converting the results of the acquisition into an estimate of the concentration of suspended sediment and integrating the results from several frequencies, it will be established the applicable frequency usage. It was concluded that a multibeam echosounder with a frequency of 450 kHz was recommended in a case study to determine the concentration of suspended sediment. This is supported by a correlation value of 89.18% or a very high correlation.

Influence of fish backbone model geometrical features on the numerical target strength of swimbladdered fish

Abstract The method of fundamental solutions has been applied to evaluate the influence of fish models geometrical features on the target strength (TS) directivity and TS frequency response of swimbladdered fish. Simplified models were considered for two fish species: gilt-head sea bream (Sparus aurata, Linnaeus 1758) and Atlantic salmon (Salmo salar, Linnaeus 1758), and different geometrical details of their morphology were studied, such as backbone presence, and its curvature or the inclusion of vertebrae modulation. Swimbladder shape and tilt, together with the inclusion of backbone (and its realistic curvature) for dorsal measurements were the most important features for proper estimation of mean TS. The estimation of mean TS is considered including the effect of fish tilt, the echosounder frequency, and the fish-to-transducer distance.

A mechanistic assessment of the potential ecological risk to seagrass meadows posed by marine echosounders

Motivated by experimental evidence, a mechanistic model is used to investigate the impact of marine echosounder transmissions on seagrass leaves. The interaction of the ultrasonic field with the seagrass is solved within the leaf cross-section. The leaf tissue is assumed to be elastic, homogeneous and isotropic, with embedded cavities representing lacunae. A standard Galerkin finite element method is employed to numerically solve the resulting equations. Natural vibration frequencies are computed for leaves of Halodule wrightii and Syringodium filiforme. Strong perturbations in the leaf structure are expected when natural and echosounder frequencies match. The results reveal that a significant number of natural frequencies are within or near the typical frequency bands of echosounder operations. In addition, stresses in the leaf tissues of seagrass at 20 m depth were computed in a typical scenario for echosounder operations. Leaf-tissue damage in S. filiforme could result under these conditions. Within the framework of the theoretical assumptions, the results highlight the potential and, until now, unnoticed risks to seagrass meadows posed by marine echosounders.

Sedimentological study of the reservoir of the Manduriacu hydroelectric project, northern Ecuador

The first years of operation of a project, especially of hydroelectric plants, requires the permanent monitoring of the critical parameters that determine the normal operation of the project, verifying or adjusting the conditions that have been established in the design and construction of the work. For the Manduriacu Project in Ecuador the critical parameter has been the volume of sediments that the Guayllabamba River carries and that reduces the useful volume of the reservoir. This study presents the evaluation of sedimentation of the reservoir for the spring 2016 to spring 2017, conducted with a variety of tools such as the marine drone Z-BOAT 1800 HS (USV), the dual frequency echo sounder ECHOTRAC CV100 DUAL, the Long Range Radio HYDROLINK, the equipment for reception of position data GNSS TRIMBLE R10, the Echosounder control software and eChart connections as well as the Hypack MAX device manager software and other technical support. From the comparative analysis of bathymetries before and after washing, it has been determined that the volume of sediment accumulated in the year has been of 4’899.263 m3 corresponding to 59.1% of the total volume of the reservoir.

Target strength of skipjack tuna (Katsuwanus pelamis) associated with fish aggregating devices (FADs)

Abstract This paper presents measures of target strength (TS; dB re 1 m2) and models of TS vs. fork length (L; cm), i.e. TS=20logL+b20, for skipjack tuna associated with fish aggregating devices (FADs) in the Central Pacific Ocean. Measurements were made using 38-, 120-, and 200-kHz split-beam echosounders on a purse-seine workboat during fishing operations. To mitigate potential bias due to unresolved targets, TS measurements were rejected if they were not simultaneously detected with multiple echosounder frequencies in approximately the same location. The filtered TS and concomitantly sampled L data were used to estimate b20 = −76, −71, and −70.5 dB for 38, 120, and 200 kHz, respectively, using the method of least squares. For comparison, quasi-independent estimates of TS and b20 were calculated from acoustic echo-integration and catch data representing entire aggregations around the FADs. The results differed by ≤1 dB for all three frequencies. The sensitivities of these results to variations in fish morphology and behaviour were explored using a simulation of TS for fish without swimbladders. The utility of the results on acoustic properties of skipjack tuna and next research steps to achieve selective fishing at FADs are discussed.

A diverse group of echogenic particles observed with a broadband, high frequency echosounder

Abstract In 1980, Holliday and Pieper stated: “Most sound scattering in the ocean volume can be traced to a biotic origin.” However, most of the bioacoustics research in the past three decades has focused on only a few groups of organisms. Targets such as small gelatinous organisms, marine snow, and phytoplankton, e.g. have been generally to be considered relatively transparent to acoustic waves due to their sizes and relatively low sound speed and density contrasts relative to seawater. However, using a broadband system (ZOOPS-O2) we found that these targets contributed significantly to acoustic returns in the 1.5–2.5 MHz frequency range. Given that phytoplankton and marine snow layers are ubiquitous features of coastal regions; this works suggests that they should be considered as potential sources of backscatter in biological acoustic surveys.

Model for the Processing and Estimation of Dual Frequency Echo Sounder Observations in Detailed Bathymetries

ABSTRACTThe dataset acquired by an echo sounder generally comprises outliers and erroneous measurements which must be automatically processed. The main aim of this research is the development and validation of an algorithm which permits detecting outliers, estimating their real value from data acquired with a multifrequency echo sounder, and verifying the measure according to the desired accuracy. The process considers the nature of the seabed and improves the density and number of lectures without globally compromising the precision, but enhancing it in areas with anomalous measurements. Twenty-nine percent of this type of measurements, which would have been otherwise rejected, have been estimated.

Classification of Southern Ocean krill and icefish echoes using random forests

Abstract Target identification remains a challenge for acoustic surveys of marine fauna. Antarctic krill, Euphausia superba, are typically identified through a combination of expert scrutiny of echograms and analysis of differences in mean volume backscattering strengths (SV; dB re 1 m−1) measured at two or more echosounder frequencies. For commonly used frequencies, however, the differences for krill are similar to those for many co-occurring fish species that do not possess swimbladders. At South Georgia, South Atlantic, one species in particular, mackerel icefish, Champsocephalus gunnari, forms pelagic aggregations, which can be difficult to distinguish acoustically from large krill layers. Mackerel icefish are currently surveyed using bottom-trawls, but the resultant estimates of abundance may be biased because of the species' semi-pelagic distribution. An acoustic estimate of the pelagic component of the population could indicate the magnitude of this bias, but first a reliable target identification method is required. To address this, random forests (RFs) were generated using acoustic and net sample data collected during surveys. The final RF classified as krill, icefish, and mixed aggregations of weak scattering fish species with an overall estimated accuracy of 95%. Minimum SV, mean aggregation depth (m), mean distance from the seabed (m), and geographic positional data were most important to the accuracy of the RF. Time-of-day and the difference between SV at 120 kHz (SV 120) and that at 38 kHz (SV 38) were also important. The RF classification resulted in significantly higher estimates of backscatter apportioned to krill when compared with widely applied identification methods based on fixed and variable ranges of SV 120–SV 38. These results suggest that krill density is underestimated when those SV-differencing methods are used for target identification. RFs are an objective means for target identification and could enhance the utility of incidentally collected acoustic data.

Shallow gas occurrence in a Brazilian ría (Saco do Mamanguá, Rio de Janeiro) inferred from high-resolution seismic data

Semi-enclosed areas, such as rías, offer low-energy sedimentary conditions, which allow the necessary environment for the deposition of clays and organic-rich deposits that generate gas through methanogenic bacteria. In Saco do Mamanguá (Rio de Janeiro, Brazil), acoustic data from low frequency echo sounder and high-resolution seismic-reflection data from chirp sonar and boomer from three surveys – February 2009 and January and June 2014 – enabled for the first time the identification of acoustic disturbances related to gas occurrences. Saco do Mamanguá hosts shallow gas accumulation throughout its entire area, expressed as acoustic turbidity, enhanced reflections or acoustic blanking, intra-sedimentary plumes and acoustic plumes. However, gas occurrence evidence decreases towards the open ocean. A lithological control occurs on the sealing layer in the inner area, whereas the smaller gas occurrence in the outer area can be due to (1) absence of a sealing layer and/or (2) the re-oxidizing of methane as more oxygenated waters are available. We propose a sequence stratigraphic model in which the gas formation is Holocene or either pre-Holocene age.

A method for identifying Sound Scattering Layers and extracting key characteristics

Summary Mid‐trophic level water‐column (pelagic) marine communities comprise millions of tonnes of zooplankton and micronekton that form dense and geographically extensive layers, known as sound scattering layers (SSLs) when observed acoustically. SSLs are ubiquitous in the global ocean, and individual layers can span entire ocean basins. Many SSLs exhibit clear diel vertical migration behaviour. Vertical migrations contribute substantially to the ‘biological pump’, such that SSLs have important global biogeochemical roles: SSLs are important conduits for vertical energy and nutrient flow. Ship‐based remote sensing of SSLs using acoustic instruments (echosounders) enables their shape and density to be quantified, but despite SSLs being discovered in the 1940s, there is no consistent method for identifying or characterising SSLs. This hampers ecological and biogeographical studies of SSLs. We have developed an automated and reproducible method for SSL identification and characterisation, the sound scattering layer extraction method (SSLEM). It functions independently of echosounder frequency and the spatial scale (vertical and horizontal) of the data. Here we demonstrate the SSLEM through its application to identify SSLs in data gathered to a depth of 1000 m using 38 kHz hull‐mounted echosounders in the south‐west Indian Ocean and Tasman Sea. SSLs were identified in the water column as horizontally extensive echoes that were above background noise. For each identified SSL, a set of 9 quantitative ‘SSL metrics’ (describing their shape, dynamics and acoustic backscattering distribution) were determined, enabling inferences to be made concerning the spatial arrangement, distribution and heterogeneity of the biological community. The method was validated by comparing its output to a set of visually derived SSL metrics that were evaluated independently by 8 students. The SSLEM outperformed the by‐eye analysis, identifying three times the number of SSLs and with greater validity; 95% of SSLs identified by the SSLEM were deemed valid, compared to 75% by the students. In the same way that data obtained from satellites have enabled the study and characterisation of global phytoplankton distribution and production, we envisage that the SSLEM will facilitate robust, repeatable and quantitative analysis of the growing body of SSL observations arising from underway‐acoustic observations, enhancing our understanding of global ocean function.

A fisheries acoustic multi-frequency indicator to inform on large scale spatial patterns of aquatic pelagic ecosystems

Fisheries acoustic instruments provide information on four major groups in aquatic ecosystems: fish with and without swim bladder (tertiary and quaternary consumers), fluidlike zooplankton (secondary consumers) and small gas bearing organisms such as larval fish and phytoplankton (predominantly primary producers). We entertain that this information is useable to describe the spatial structure of organism groups in pelagic ecosystems. The proposal we make is based on a multi-frequency indicator that synthesises in a single metric the shape of the acoustic frequency response of different organism groups, i.e. the dependence of received acoustic backscattered energy on emitting echosounder frequency. We demonstrate the development and interpretation of the multi-frequency indicator using simulated data. We then calculate the indicator for acoustic water-column survey data from the Bay of Biscay and use it to create reference maps for the spatial structure of the four scattering groups as well as their small scale spatial variability. These maps provide baselines for monitoring future changes in the structure of the pelagic ecosystem.

Benthic habitat mapping: Concerns using a combined approach (acoustic, sediment and biological data)

Near-shore benthic biotopes, in the 2–30 m depth range, were analyzed and mapped using a combination of biological, sedimentary and acoustic data to interrogate the utility of these methods. The surveyed area covered approximately 270 km 2, along 80 km of the Southeast coast of Portugal. The acoustic data were acquired with a single beam ground discrimination system (QTC VIEW, Series V), connected to a dual frequency echo sounder (50 and 200 kHz). Sediment grain size and macrofaunal community data were obtained by grab sampling at 88 sites. The sedimentary and the biological data were subjected to classification and ordination analysis and the acoustic data obtained with the two frequencies were analyzed individually with the QTC IMPACT (v3.4) software and classified into acoustic classes. The affinity groups obtained with the three datasets were mapped using a Geographic Information System. The study area showed an inshore-offshore pattern with sands ranging from very fine to very coarse, revealing a sediment distribution in the near-shore shelf comprised by two main areas; a very fine/fine sand area up to 7–10 m depth and a very coarse/coarse sand area offshore. In turn, the biological data also showed an inshore-offshore pattern, identifying two main biological assemblages: fine/very fine sand and coarse/very coarse sand communities. Furthermore, the biological pattern clearly reflected the hydrodynamic conditions of the area, from highly hydrodynamic shallow depth to the calmer deeper areas. Accordingly, species richness and densities were lower at shallow areas than at deeper depth. The acoustic data identified the prevailing biosedimentary gradients along the inshore-offshore direction. Overall, the results obtained here showed that the acoustic system could identify two main areas based on sediment grain size, which closely related to the two main biological communities (groups A and B). This study indicates the importance of combining several layers of information in order to increase the spatial resolution of the main biotope distribution (validating the acoustic data) and detail their sub-divisions (ground-truth sampling). Furthermore, we show that designing ground-truth samples on the basis of the acoustic diversity data allows the biological data to “speak” for itself. In general, acoustic techniques are used as a means to optimize ground-truth sampling. This will only be valid if acoustics can capture all the essence of biotope heterogeneity. If this is not the case, special attention should be given when using broad scale methods (acoustic remote sensing) devoted to biotope mapping.

High-resolution marine magnetic surveys for searching underwater cultural resources

Recently two marine magnetic surveys, combined with the use of a multi-beam sonar (Kongsberg Marittime EM 300 multibeam: 30 KHz frequency echosounder for hydrographic purposes; acoustic lobe composed of 128 beams able to cover a 150° sector) a side-scan sonar (Simrad MS 992 dual-frequency sidescan sonar with echo sounder transducers 150 Hz and 330 KHz) and a Remote Operated Vehicle (ROV – a mobile tools used in environments which are too dangerous for humans), were executed in two sites respectively in the Ligurian Sea and the Asinara Gulf. The aim of these investigations was to test modern instrumentations and set new working procedures for searching underwater cultural resources. The collected and processed magnetic data yielded very satisfactory results: we detected submerged and buried features of cultural interest at both sites, at depths of 40 m and 400 m respectively.

Variations in echosounder–transducer performance with water temperature

Abstract Demer, D. A., and Renfree, J. S. 2008. Variations in echosounder–transducer performance with water temperature. – ICES Journal of Marine Science, 65: 1021–1035. Electro-acoustic transducers are central components of multifrequency echosounders used in remote-target identification and acoustic surveys for fish and zooplankton. Appreciable changes in echosounder system gains can result from shifts in transducer frequency responses with water temperature. Because it is standard practice to calibrate echosounder systems for fisheries surveys in one environment and apply the resulting gains to interpret data collected over the range of sea temperatures encountered during a survey, the results may be biased. Such biases may be different for estimates derived from each echosounder frequency. In moving to quantify and mitigate these effects, the performances have been measured for ten commonly used survey transducers in water temperatures ranging from ∼1°C to 18°C, using three techniques. Results show that the transducer impedances all change with temperature, potentially changing the signal-to-noise ratio from 5 to >20 dB. The resonance frequencies and quality factors also change with temperature, ranging from ∼0.2% to 2.8% and 2.5% to >130%, respectively. Corresponding directly to changes in the echosounder gains, the transmitting-current and receiver-voltage responses changed 1 dB or less with temperature, except for the Simrad ES120-7, which showed a 2 dB increase. Generally, the magnitudes of frequency-dependent biases in echosounder-system gains depend primarily on the temperature-dependent performances of the survey transducers, the range of temperatures encountered, and whether the operational frequencies are less or greater than the resonance frequency.

Echo‐sounding can discriminate between fish and macroinvertebrates in fresh water

Summary1. Acoustic scattering from fish and macroinvertebrates was studied in a boreal Finnish lake at three echosounder frequencies (38, 120 and 200 kHz). Split‐beam transducers with partly overlapping 7° beams were employed. Acoustic, fish and invertebrate sampling were undertaken simultaneously. Vertical gradients of temperature and oxygen concentration were measured during the exercise.2. At all frequencies, a narrow scattering layer coincided with the thermocline. At 38 kHz, fish were detected well with practically no reverberation from invertebrates while 200 kHz detected both fish and invertebrates.3. Minor differences in the magnitude of acoustic scattering from fish were found between frequencies and between depth layers, but scattering at different frequencies was correlated at all depths. Acoustic scattering and fish density indices from trawl catches, consisting mostly of smelt (Osmerus eperlanus) (97%) and vendace (Coregonus albula) (3%), were significantly correlated.4. Acoustic scattering from invertebrates increased with sound frequency. Correlation analysis suggested that the invertebrate scattering was mostly induced by Chaoborus flavicans. A low frequency is recommended for estimating fish abundance without bias from reverberation induced by invertebrate scattering. Although fish and invertebrates can also be successfully discriminated at a single frequency by thresholding and cross filtering, the combination of a low and a high frequency is a more robust tool for effective fish‐invertebrate discrimination.

Variations of echosounder transducer performance versus water temperature

Electroacoustic transducers are central components of multi-frequency echosounders used in remote target identification and acoustical surveys for fish and zooplankton. While these transducers, constructed from multiple materials such as piezoelectric ceramics, polyurethanes, and metals, are designed to be insensitive to performance changes versus time, pressure, and temperature, it has been shown that appreciable changes in echosounder system gains result from shifts in transducer resonant frequency, quality factor, and electrical impedance versus water temperature. Because it is standard practice to calibrate echosounder systems in one environment and apply the resulting gains to interpret data collected over a range of sea temperatures, the survey results may be significantly biased. The bias may be different for estimates derived from each echosounder frequency. The magnitudes of these frequency-dependent biases depend upon the temperature-dependent performances of the survey transducers, and the range of temperatures encountered during the survey. Here, the performance has been measured for ten commonly used survey transducers versus water temperature ranging from 1 to 18<th>°C, using four different techniques. Temperature-dependent system gain corrections are proposed that can be used to minimize bias in multifrequency target identifications and biomass estimations. [Funding was provided by the NMFS Advanced Sampling Technology Working Group.]

Acoustic scattering from a larval insect ( Chaoborus flavicans) at six echosounder frequencies: Implication for acoustic estimates of fish abundance

Acoustic scattering from Chaoborus flavicans larvae was studied in a virtually fishless lake using multi-frequency split-beam echosounders with 7° partly overlapping acoustic beams. Six echosounders simultaneously operating at 38, 70, 120, 200 and 364 and 710 kHz were employed. Zooplankton sampling, using a Schindler-Patalas trap, was used to identify the nature and composition of acoustic scatterers at five different depths and to establish a correlation between observed and acoustic volume density of Chaoborus. Chaoborus showed high scattering at 200 kHz (NASC > 30,000 m 2 nmi −2, Δ d = 10 m), but was virtually “invisible” at 38 kHz (NASC < 10 m 2 nmi −2, Δ d = 10 m). The remaining frequencies produced intermediate responses, resulting in a distinctive signature of the frequency response curve that ranged from Rayleigh to geometrical scattering, with a resonance around 200 kHz. The mean target strength ranged from −60.5 to −64.3 dB re 1 m 2 depending on body length. The correlation between the volume density of Chaoborus established from the direct sampling and the acoustic recordings was poor ( r 2 < 0.37), but agreement was found between density maxima. Although other zooplankton species were present and at higher densities, Chaoborus were identified as the source of the acoustic scattering. Chaoborus can represent a major error in acoustic estimates of fish densities obtained at higher echosounder frequencies, but not at lower frequencies (e.g. 38 kHz). The distinctive acoustic signature of Chaoborus demonstrates the usefulness of multi-frequency acoustics for species identification and discrimination. Acoustic methods selectively assess vertical distribution maxima for Chaoborus and size, but cannot reliably estimate abundance.

Acoustic Detection of Methane Plumes

To understand the complexity of the global methane cycle with its relation to climate change, it is necessary to identify relevant methane sources and to assess their environmental functions. The contribution of the various marine methane sources to the global cycle is only poorly understood so far. Methods are needed to survey efficiently methane release from the sea floor. Remote techniques are favoured since they allow a survey of larger areas than, for example, sediment sampling. Low frequency echosounders, as used in fishery, were recognized to be appropriate tools for plume detection and are hoped to be the key for efficient plume mapping and gas quantification. Here, we present methane plumes from three different marine environments.

Adaptive neuro-fuzzy and fuzzy decision tree classifiers as applied to seafloor characterization

This paper investigates the influence of various backscattered echo parameters collected at an echosounder frequency of 200 kHz on the performance of the neuro-fuzzy and fuzzy decision tree classifiers of a seabed. In particular, the wavelet coefficients of the bottom echo were investigated along with other echo parameters like energy, amplitude, slope of the falling part of the echo, etc. The data were processed in an Adaptive Neuro Fuzzy Inference System (ANFIS), which was implemented in two multistage structures, viz.; Incremental Fuzzy Neural Network (IFNN) and Aggregated Fuzzy Neural Network (AFNN). The number of input parameters for the networks was reduced by using the Principal Component Analysis (PCA). A fuzzy decision tree algorithm was developed and used directly (without PCA data reduction) in the classification procedure utilizing the same data. The performances of both approaches were analyzed and compared.

An operational system for processing and visualizing multi-frequency acoustic data

Calibrated and digitized data from two or more discrete echosounder frequencies can be combined for the purpose of separating and extracting the acoustic scattering from zooplankton and fish in mixed recordings. This method is also useful for quantifying the relative contribution of each frequency to the total acoustic-backscattering when scrutinizing records in large-scale, acoustic surveys. Echosounder hardware requirements are defined which would permit the ideal extraction of such information. These include calibration, transducer specification, pulse resolution and digital representation of the signals. During this initial study a special version of the Simrad EK500 multi-frequency, split-beam echosounder and the Bergen Echo Integrator (BEI) post-processing system were used. The echosounder transmitted pulses simultaneously at four frequencies, 18, 38, 120 and 200 kHz and transferred the received signals to the post-processing system in calibrated, raw, digitized format. Methods are described for echogram manipulation and for the construction of new, synthetic, combined-frequency [c(f)] echograms. Examples of extracted scattering information from mixed layers of fish and small scattering-organisms, such as copepods and euphausiids, are shown, and the potential of the method is discussed.