DOI: 10.1594/pangaea.889317
title: AUV MARUM-SEAL Dive 70: High Resolution Bathymetry and Backscatter of Chapopote Asphalt Volcano

description: Abstract & Purpose of the cruise M114At the so-called asphalt volcanoes in the southern Gulf of Mexico heavy oil is seeping at the seafloor where it remains as asphalt deposits. Discovered and preliminarily surveyed during SO174 and M67/2 expeditions, these sites are subject for detail studies during M114 focusing on mapping with autonomous underwater vehicle (AUV MARUM-SEAL), deep-towed sidescan sonar (DTS-1), sediment echosounder (Parasound), multibeam echosounder (EM122), and remotely operated vehicle MARUM-ROV Quest. The overarching objective is to better understand the impact, fate, and decay rates of oil in the deep-sea environment.Heavy oil and gas bubbles are emitted from the 1200 to 2900 m deep seafloor in the hy-drocarbon province Campeche Knolls in the southern Gulf of Mexico. The viscous heavy oil flows across the seafloor, loses volatile compounds, solidifies, and is converted to asphalt with time. Due to the fact that the heavy oil remains at the seafloor, these sites are natural laboratories to study the impact of oil on deep-sea ecosystems, and the time scales of oil and asphalt degradation. These subjects are very timely, and can help understanding effects of deep water oil spills as caused by the 2010 Deepwater Horizon accident in the northern Gulf of Mexico. We propose to study the extent of oil emissions and asphalt deposits using sidescan sonar and to investigate them further employing ROV Quest. A further major topic of the proposed cruise addresses the question whether or not methane can reach the sea surface and may contribute to the pool of greenhouse gases. The fact that seepage of oil-coated gas bubbles leads to oil slicks at the sea surface and enhanced methane concentrations was recently shown in the north-ern Gulf. It can be assumed that similar efficient transport processes for methane exists in the area of the Campeche Knolls, where oil slicks have been observed in association with about ~30 individual seafloor structures.
description: Description of the AUV platform and the payload:The company International Submarine Engineering (I.S.E.) built the AUV MARUM-SEAL in 2005-2006 as #5 of its Explorer-Class AUVs. It is nearly 5.75 m long, with a diameter of 0.73 m and a weight of 1.35 tons in air. The AUV consists of a modular atmospheric pressure hull, designed as two hull segments and a front and aft dome. Inside the pressure hull, the vehicle control computer (VCC), the payload control computer (PCC), eight lithium batteries and spare room for additional "dry" payload electronics are located. The inertial navigation system IXBlue PHINS and the KONGSBERG multibeam-processor (VxWorks computer) are located as dry payload here. The tail and the front section, built from GRP-material, are flooded wet bays. In the tail section the motor, beacons for USBL, RF-radio, Flashlight, IRIDIUM antenna and DGPS antenna are located. The Seabird SBE 49 CTD, the Sercel MATS 200 acoustic modem, the Doppler Velocity Log (DVL, 300kHz), KONGSBERG Pencil beam (675kHz) as obstacle avoidance, the KONGSBERG EM2040 (200,300, 400kHz) implemented in 2014, the PAROSCIENTIFIC pressure-sensor and the BENTHOS dual frequency (100/400kHz) side scan sonar can be mounted as optional payload in the custom made aluminium front section. The SEAL AUV has a capacity of approx. 15,4 KWh main energy, enabling the AUV to conduct approx. 65 km mission-track lengths. However, mission-track lengths had to be reduced during M114 due to water depths of about 4km and the more energy consuming EM2040 MBES, compared to former cruises. For security aspects, several hard- and software mechanisms are installed on the AUV to minimize the risk of malfunctioning, damage, and total loss. More basic features are dealing with fault response tables, including an emergency drop weight, either released by user or completely independent by AUV time-relays itself. MARUM put special emphasis on an open architecture in terms of hard- and software design of the AUV, in order to guarantee modular and flexible vehicle operations. Therefore, the VCC is based largely on industrial electronic components and compact-PCI industrial boards and only few proprietary hardware boards have been implemented. The software is completely built on QNX 4.25 - a licensed UNIX derivate, open to large extents for user modifications. The payload PC is built on comparable hardware components, but running either with OS Windows and/or OS Linux. On the support vessel, the counterpart to the VCC is located on the surface control computer (SCC). It is designed as an Intel based standard PC, also running with same QNX OS and a Graphic User Interface (GUI) to control and command the MARUM SEAL AUV. Payload during M114 CTD Seabird SBE49 Fastcat. Last calibration from 2010. KONGSBERG EM2040 setup (400kHz dive 67-70); Range of 150m (400kHz); recording of water column data was on during all dives. Acquisition of Multibeam-Echsounder (MBES) Data A MBES system on an AUV requires a number of auxiliary sensors for position, motion, sound velocity and depth of the vehicle. The INS PHINS delivers, based on complex Kalman filters, a position and attitude data for the AUV platform, while sound velocity (SV) is calculated utilizing an UNESCO SV equation on CTD sensor measurements. The pressure to depth calculation is based on a simplified equation, depending mainly on the latitude of the area of interest and an average water depth of the area investigated, and is used as static factor for the recalculation. Therefore, the recorded data could be enhanced by post-processing of the pressure data with the UNESCO pressure to depth equation. The EM2040 system itself is controlled by the VCC of the AUV and records data directly on the control unit of the EM2040 (VxWorks computer) as *.all files including the option to store water column data. The PAROSCIENTIFIC depth sensor presented a large number of noise spikes varying between +/- 1 meter. The CTD depth data was a lot smoother but presented a non-linear depth drift that generated some unreal long scale oscillations. Due to this the PARASCIENTIFC was preferred and re-processed. Description of data processing: The Seafloor-Imaging group of MARUM, responsible person Christian dos Santos Ferreira and CI Paul Wintersteller (seafloor-imaging@marum.de), conducted postprocessing and products of the EM2040 data of AUV dives 67 to 70. The MB-system suite (Caress, D.W., and D.N. Chayes, MB-System Version 5, an open source software distributed from the MBARI and L-DEO web sites, 2000-2017) as well as QPS Fledermaus™ were utilized for this purpose. A tide correction was applied, based on the Oregon State University (OSU) tidal prediction software (OTPS) that is retrievable through MB-System. A pitch correction was not required other than a roll correction of 0.05°, observed in roll-calibration lines taken at the beginning of dive 67. Bathymetric data has been manually cleaned for existing artefacts with MB-System's mbeditviz tool. No SV profile correction was applied. MBnavadjust, an MB-system tool, allowed rectifying for position-shifts due to e.g. DVL drift. After receiving it's last DGPS signal when starting to dive, the DVL (Doppler Velocity Log) is next to the propeller rotation the only consecutive information the INS PHINS uses to calculate the absolute position of the AUV during mission-mode. Since the DVL delivers only velocity through water until 200m above sea floor, the AUVs position while diving down to the sea floor is rather relative and exposed to potential currents of the different water masses. Due to the high frequency noise in the PARASCIENTIFIC data (depth sensor) a despike filter was applied before the depth sensor got smoothed by a 4 seconds window. Without that the amount and vertical scale of the spikes would influence the depth sensor smoothing generating some short scale (1-2 seconds) false depth oscillations manifested as woobling and heave artifacts. The open source package "R" was utilized using the function called "despike" provided by package "oce" for analysis of oceanographic data (and time series). The MBnavadjust corrected navigation is therefore evaluated with respect to plausibility e.g. to avoid strong, unrealistic tying or bending of the navigation track. A two-three steps workflow has been established correcting thirst every AUV dive within itself (line by line ties) while the second step includes referencing to the shipside bathymetry and a potential third step the combination of different AUV dives next to each other. NetCDF (GMT) grids of the product and the statistics were created using mbgrid. No total propagated uncertainty (TPU) has been calculated to gather vertical or horizontal accuracy. The currently published bathymetric grid of the cruise has a resolution of 10 m. A higher resolution up to 1 m is achievable and used for current scientific investigations. The grid extended with _num represents a raster dataset with the statistical number of beams/depths taken into account to create the depth of the cell. The extended "_sd"-grid contains the standard deviation for each cell. Grid naming and explanation: Term1_Term2_Term3_Term4_Term5_Term6/optTerm7/optTerm8 e.g.: 2015_M114_EM2040_AUV_D70_A2F1C7E10m/_num/_sd Term1: Year of the survey Term2: Cruise Term3: MBES sensor Term4: Carrier e.g. AUV Term5: Dive# (carrier internal) Term6: MB-System syntax for A2 (Bathy), A3 (Amplitude), A4 (Beam Time Series); E10m for 10m grid cell resolution Term7: Grid showing the # of beams/points within a grid cell Term8: Grid showing the standard deviation for each grid cell Size/Amount of data: NetCDF Km of hydroacoustic survey - 34.85 km Storage &lt;10MB for all the datasets