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The sediment backscatter strength measured by multibeam echosounders is a key feature for seafloor mapping either qualitative (image mosaics) or quantitative (extraction of classifying features). An important phenomenon, often underestimated, is the dependence of the backscatter level on the azimuth angle imposed by the survey line directions: strong level differences at varying azimuth can be observed in case of organized roughness of the seabed, usually caused by tide currents over sandy sediments. This paper presents a number of experimental results obtained from shallow-water cruises using a 300-kHz multibeam echosounder and specially dedicated to the study of this azimuthal effect, with a specific configuration of the survey strategy involving a systematic coverage of reference areas following “compass rose” patterns. The results show for some areas a very strong dependence of the backscatter level, up to about 10-dB differences at intermediate oblique angles, although the presence of these ripples cannot be observed directly—neither from the bathymetry data nor from the sonar image, due to the insufficient resolution capability of the sonar. An elementary modeling of backscattering from rippled interfaces explains and comforts these observations. The consequences of this backscatter dependence upon survey azimuth on the current strategies of backscatter data acquisition and exploitation are discussed.

The calibration of multibeam echosounders for backscatter measurements can be conducted efficiently and accurately using data from surveys over a reference natural area, implying appropriate measurements of the local absolute values of backscatter. Such a shallow area (20-m mean depth) has been defined and qualified in the Bay of Brest (France), and chosen as a reference area for multibeam systems operating at 200 and 300 kHz. The absolute reflectivity over the area was measured using a calibrated single-beam fishery echosounder (Simrad EK60) tilted at incidence angles varying between 0° and 60° with a step of 3°. This reference backscatter level is then compared to the average backscatter values obtained by a multibeam echosounder (here a Kongsberg EM 2040-D) at a close frequency and measured as a function of angle; the difference gives the angular bias applicable to the multibeam system for recorded level calibration. The method is validated by checking the single- and multibeam data obtained on other areas with sediment types different from the reference area.

The increased use of backscatter measurements in time series for environmental monitoring necessitates the comparability of individual results. With the current lack of pre-calibrated multibeam echosounder systems for absolute backscatter measurement, a pragmatic solution is the use of natural reference areas for ensuring regular assessment of the backscatter measurement repeatability. This method mainly relies on the assumption of a sufficiently stable reference area regarding its backscatter signature. The aptitude of a natural area to provide a stable and uniform backscatter response must be carefully considered and demonstrated by a sufficiently long time-series of measurements. Furthermore, this approach requires a strict control of the acquisition and processing parameters. If all these conditions are met, stability check and relative calibration of a system are possible by comparison with the averaged backscatter values for the area. Based on a common multibeam echosounder and sampling campaign completed by available bathymetric and backscatter time series, the suitability as a backscatter reference area of three different candidates was evaluated. Two among them, Carré Renard and Kwinte, prove to be excellent choices, while the third one, Western Solent, lacks sufficient data over time, but remains a valuable candidate. The case studies and the available backscatter data on these areas prove the applicability of this method. The expansion of the number of commonly used reference areas and the growth of the number of multibeam echosounder controlled thereon could greatly contribute to the further development of quantitative applications based on multibeam echosounder backscatter measurements.

A rotating, acoustic gas bubble detector, BOB (Bubble OBservatory) module was deployed during two surveys, conducted in 2009 and 2011 respectively, to study the temporal variations of gas emissions from the Marmara seafloor, along the North Anatolian Fault zone. The echosounder mounted on the instrument insonifies an angular sector of 7° during a given duration (of about 1 h). Then it rotates to the next, near-by angular sector and so forth. When the full angular domain is insonified, the “pan and tilt system” rotates back to its initial position, in order to start a new cycle (of about 1 day). The acoustic data reveal that gas emission is not a steady process, with observed temporal variations ranging between a few minutes and 24 h (from one cycle to the other). Echo-integration and inversion performed on the acoustic data as described in the companion paper of Leblond et al. (Mar Geophys Res, 2014 ), also indicate important variations in, respectively, the target strength and the volumetric flow rates of individual sources. However, the observed temporal variations may not be related to the properties of the gas source only, but reflect possible variations in sea-bottom currents, which could deviate the bubble train towards the neighboring sector. During the 2011 survey, a 4-component ocean bottom seismometer (OBS) was co-located at the seafloor, 59 m away from the BOB module. The acoustic data from our rotating, monitoring system support, but do not provide undisputable evidence to confirm, the hypothesis formulated by Tary et al. ( 2012 ), that the short-duration, non-seismic micro-events recorded by the OBS are likely produced by gas-related processes within the near seabed sediments. Hence, the use of a multibeam echosounder, or of several split beam echosounders should be preferred to rotating systems, for future experiments.

Seafloor backscatter mosaics are now routinely produced from multibeam echosounder data and used in a wide range of marine applications. However, large differences (>5 dB) can often be observed between the mosaics produced by different software packages processing the same dataset. Without transparency of the processing pipeline and the lack of consistency between software packages raises concerns about the validity of the final results. To recognize the source(s) of inconsistency between software, it is necessary to understand at which stage(s) of the data processing chain the differences become substantial. To this end, willing commercial and academic software developers were invited to generate intermediate processed backscatter results from a common dataset, for cross-comparison. The first phase of the study requested intermediate processed results consisting of two stages of the processing sequence: the one-value-per-beam level obtained after reading the raw data and the level obtained after radiometric corrections but before compensation of the angular dependence. Both of these intermediate results showed large differences between software solutions. This study explores the possible reasons for these differences and highlights the need for collaborative efforts between software developers and their users to improve the consistency and transparency of the backscatter data processing sequence.

Claude-Marie Bonnefont, nommé en 1796, occupe les fonctions de commissaire du Directoire dans le département de la Vienne jusqu'au début du Consulat. Ce modéré, compétent et actif, se révèle un bon administrateur. Toujours présent lors des réunions de l'administration départementale, il est en relation étroite avec les municipalités de canton, accélère la conscription, veille à la sécurité des routes et lutte contre les « brigands royaux ». Dans les rapports qu'il adresse aux ministres, il rend compte de l'opinion publique dont il fait une présentation optimiste alors qu'il y a des signes d'agitation. Après le coup d'État du 18 Fructidor an V, Bonnefont devient la cible préférée des anciens Jacobins. Il manque d'être destitué, mais réussit à se maintenir en place. Sous le Consulat et l'Empire, il sera secrétaire général de la préfecture de la Vienne. Claude-Marie Bonnefont, Commissaire of the Directory in the Vienne Department Claude-Marie Bonnefont was appointed commissaire in the Vienne department in 1796 and continued in this post until the Consulate. A moderate man, active and effective, he proved a good administrator. He attended all the meetings of the department council, kept in close touch with the canton municipalities, hastened conscription, kept an eye on the safety of the highways and led the fight against the « royal brigands ». In his reports to ministers, he gave accounts of public opinion which were always optimistic when there were signs of unrest. After the coup of 18 Fructidor Year V, Bonnefont became the whipping-boy of the former Jacobins. He came close to dismissal, but succeeded in keeping his post. Under the Consulate and Empire, he was permanent under-secretary to the prefect of the Vienne.

A new seafloor mapping system was installed on E/V Nautilus in early 2013, revolutionizing the way the ship explores the ocean. Not only can the Kongsberg EM 302 multibeam echosounder (Figure 1) create maps of the seafloor, it can also provide information on bottom type from backscatter data, similar to the side-scan sonar Diana. Additionally, the multibeam can be used to locate plumes in the water column created by hydrothermal vents or gas or oil seeps. Together with the new Knudsen 3260 subbottom profiler, these acoustic instruments will provide improved maps of the seafloor to help scientists choose dive locations and to improve situational awareness for navigators and ROV pilots. Data are archived at NOAA's National Geophysical Data Center, allowing anyone to obtain raw or gridded seafloor bathymetric data collected during Nautilus's exploratory cruises.

Claude-Marie Bonnefont, nommé en 1796, occupe les fonctions de commissaire du Directoire dans le département de la Vienne jusqu'au début du Consulat. Ce modéré, compétent et actif, se révèle un bon administrateur. Toujours présent lors des réunions de l'administration départementale, il est en relation étroite avec les municipalités de canton, accélère la conscription, veille à la sécurité des routes et lutte contre les « brigands royaux ». Dans les rapports qu'il adresse aux ministres, il rend compte de l'opinion publique dont il fait une présentation optimiste alors qu'il y a des signes d'agitation. Après le coup d'État du 18 Fructidor an V, Bonnefont devient la cible préférée des anciens Jacobins. Il manque d'être destitué, mais réussit à se maintenir en place. Sous le Consulat et l'Empire, il sera secrétaire général de la préfecture de la Vienne. Claude-Marie Bonnefont, Commissaire of the Directory in the Vienne DepartmentClaude-Marie Bonnefont was appointed commissaire in the Vienne department in 1796 and continued in this post until the Consulate. A moderate man, active and effective, he proved a good administrator. He attended all the meetings of the department council, kept in close touch with the canton municipalities, hastened conscription, kept an eye on the safety of the highways and led the fight against the « royal brigands ». In his reports to ministers, he gave accounts of public opinion which were always optimistic when there were signs of unrest. After the coup of 18 Fructidor Year V, Bonnefont became the whipping-boy of the former Jacobins. He came close to dismissal, but succeeded in keeping his post. Under the Consulate and Empire, he was permanent under-secretary to the prefect of the Vienne.

Claude-Marie Bonnefont, nommé en 1796, occupe les fonctions de commissaire du Directoire dans le département de la Vienne jusqu'au début du Consulat. Ce modéré, compétent et actif, se révèle un bon administrateur. Toujours présent lors des réunions de l'administration départementale, il est en relation étroite avec les municipalités de canton, accélère la conscription, veille à la sécurité des routes et lutte contre les « brigands royaux ». Dans les rapports qu'il adresse aux ministres, il rend compte de l'opinion publique dont il fait une présentation optimiste alors qu'il y a des signes d'agitation. Après le coup d'État du 1 8 Fructidor an V, Bonnefont devient la cible préférée des anciens Jacobins. Il manque d'être destitué, mais réussit à se maintenir en place. Sous le Consulat et l'Empire, il sera secrétaire général de la préfecture de la Vienne. Jean-Marie Augustin, Claude-Marie Bonnefont, Commissaire of the Directory in the Vienne Department Claude-Marie Bonnefont was appointed commissaire in the Vienne department in 1796 and continued in this post until the Consulate. A moderate man, active and effective, he proved a good administrator. He attended all the meetings of the department council, kept in close touch with the canton municipalities, hastened conscription, kept an eye on the safety of the highways and led the fight against the « royal brigands ». In his reports to ministers, he gave accounts of public opinion which were always optimistic when there were signs of unrest. After the coup of 18 Fructidor Year V, Bonnefont became the whipping-boy of the former Jacobins. He came close to dismissal, but succeeded in keeping his post. Under the Consulate and Empire, he was permanent under-secretary to the prefect of the Vienne.