The major outcome from this study has been to give the marine acoustics community a voice to identify the major concerns and limitations they have with wanting to employ backscatter data in their research. There was no doubt in the community that these data will continue to be useful, and, in time, better understood which in turn will add inherent value to backscatter data.
Stability and accuracy remain the top issues for backscatter data and its derived products. We analyzed the requirements for the users using image processing and signal processing approaches. Although the two user groups differed in their approach to utilize the backscatter data and the final products that they developed, stability and accuracy issues equally affected both groups.
These top issues are related to the greatest concerns to the backscatter user’s communities—that being the ‘lack of calibration required for optimizing backscatter data’, the lack of standardization methods available for referencing and the ongoing struggles with the large data volumes (relevant to both data storage and time required for processing). Issues of standardized interpretation of backscatter within and between surveys for both geological and biological interpretation demand a pathway forward for new
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workflows for MBES backscatter processing. Many applications of habitat mapping make use of some form of classification or modelling and will often integrate backscatter data directly or indirectly in this process. Standardized data are essential for this workflow. Data quality and signal levels/properties that differ between surveys instruments within a same study area lead to poor classification/modelling results and weaken the usefulness of backscatter data for habitat mapping.
In the past where several frequencies of MBES have been used within a mapping area, the interpreter would be aware of the differences in penetration and scattering mechanisms arising from e.g., shallow water and deep water echosounders, since these will not necessarily affect the dB levels, but would have been referred to in order to aid in backscatter interpretation. Today the traditional qualitative methods are becoming obsolete as the data sets become larger and image-processing techniques begin to offer comparatively consistent and improved interpretation. In contrast to only differentiating the major sediment classes of sediment type, benthic habitat mapping (i.e., mapping that integrates the biological properties of the seabed with the seabed facies) has benefited in particular from advances in automated methods for processing BS data. Partly this may be because it is often not feasible to collect biological data over the same vast expanse as surficial sediment backscatter data, and that inference and extrapolation are required from a very small and well-understood area of seabed, which was led by predictive mapping methods based on quantitative modelling. We now see the growth in automated classification and extraction of quantitative descriptors of the backscatter amplitude, or signal properties being used to interpret and classify BS data e.g., texture measures [63,96] and estimates of geotechnical properties [15].
The user survey did not adequately capture from the user community as whether the BS data they employed was collected ‘in-house’ or obtained through contract work, or from incidental backscatter data collected as ‘by catch’ during bathymetric surveys. Although the backscatter data acquisition and processing techniques are at their nascent, in the last 10 years they have been developed to an extent where with additional and appropriate resources (trained personnel), equipment (calibrated sonars) and diligence
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(times of surveys, environmental considerations) reasonable and useful backscatter products can be generated.
The question remains as to whether these additional resources are justified where the primary purpose of the multibeam survey is not to collect backscatter but to collect bathymetric data. It can be argued confidently that the majority of multibeam sonar users emphasize bathymetric data collection as compared to seafloor and water column backscatter. In areas where no mapping data exist, bathymetric data are as equally important to satisfy the needs of different applications. In rough weather, for example, where the bathymetric data quality can still be acceptable should these surveys stop collecting data if the backscatter quality is being compromised? A major issue for maintaining the backscatter data quality is that there are no ‘easy to use’ quality metrics for backscatter data. Whilst bathymetric data quality in terms of stability and accuracy can be confirmed using a host of available methods (e.g., patch test, lead line); the backscatter comparison is challenging as the parameters that control the seafloor backscatter quality are difficult to quantify or are ill-defined and there are no easy-to-deploy at-sea methods that can provide confidence during the acquisition that high-quality backscatter data are being acquired.
The metrics used to assess the quality of backscatter data currently include visual assessment of artefacts/ system related effects as well as comparison of the backscatter data with ground truth data. The user survey identified videos, photographs, sediment grabs and real-time observations of seafloor geology and biological cover recorded from a towed camera sled as the most commonly used ground truth methods. The seafloor backscatter data provides the geo-acoustical properties of the seafloor and traditional ground truthing data may not provide explanation for the variations in the seafloor backscatter [86]. In-situ ground truthing in terms of sediment acoustic properties have been proposed [97] but so far these ground truthing methods have not gained widespread acceptance mostly due to the fact that users are more interested in the geophysical properties that they can infer from the geo-acoustical observations (i.e., seafloor backscatter). The linkages between the geo- acoustical and geophysical properties is an active field of research and beyond the scope of this paper but it is important to realize that seafloor backscatter is not capable of
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providing all the geophysical properties that a user may want to obtain and therefore combination of ground truthing data and seafloor backscatter should be dealt with due caution to avoid over interpretation of the seafloor backscatter data. To appropriately use backscatter data (both for image processing and signal processing), critical information about data collection procedures, data processing steps, lineage of applied corrections, and environmental conditions need to be carried forward to the backscatter products. Currently this remains a challenge but this can be achieved by developing a framework for establishing backscatter metadata standards.