COVIS DATA DESCRIPTION Description of COVIS COVIS (Cabled Observatory Vent Imaging Sonar) employs a modified Reson 7125 SeaBat multibeam sonar to image and quantify hydrothermal discharge. The COVIS sonar transducers are mounted atop a 4.2-m tower and have three independent degrees of freedom (pan, tilt, roll). The electronics and data-handling hardware are contained in two pressure vessels near the base of the tower. COVIS is specifically designed to (a) acquire water-column multi-beam backscatter data for imaging hydrothermal plumes and measuring their vertical velocity and heat flux and (b) acquire acoustic backscatter from the seafloor covered by diffuse hydrothermal discharge. COVIS makes three types of sonar measurements: (1) IMAGING, which provides 3D images of volume scattering strength with relatively little processing, (2) DOPPLER, which uses the full waveform data to extract fluid line-of-sight velocities, and (3) DIFFUSE-FLOW IMAGING in which the change in seafloor echos from separate transmission is used to map and quantify diffuse-flow activity. The Reson Seabat has a single EM7216 dual-frequency receive array, complemented by two projectors: the TC2162 wide-beam projector operates at both 200 and 400 kHz, while the TC2160 fan-beam projector operates only at 400kHz. The receiver array, with digital beamforming, provides 128 beams with a 3-dB azimuthal width of 1.0º at 200 kHz and 256 beams of 0.5º width at 400 kHz. In both cases, a 128º sector is covered. If necessary, the sensor head on COVIS can be panned to provide a total coverage of up to 256º of azimuth by taking multiple 128º sector scans in sequence. The system’s total range of motion in yaw is limited by the system cabling. For the diffuse-flow measurements, COVIS operates at 200 kHz with the TC2162 projector covering a wide azimuthal sector (130º 3-dB beamwidth) with 20º 3-dB beamwidth in elevation. The sonar head is held in a fixed, negative (tilted downward) elevation. Both Doppler and Imaging measurements (which target the plumes rising above focused discharges) use the TC2160 projector at 400 kHz with a transmit beam that is wide in the azimuthal coordinate (130º 3-dB beamwidth) and narrow in the elevation coordinate (1º 3-dB beamwidth). In these cases, COVIS’ sensor array is swept through a range of elevations in 1º increments. Overall, the Imaging measurements use a similar setup to the Doppler, but with a narrower pulse and fewer pings per elevation to get a higher resolution image. In the previous deployment at Grotto vent on the Ocean Network Canada Neptune cabled observatory, a single data collection sequence included one set of each of the three modes (imaging, Doppler, diffuse) with the overall sequence requiring approximately 1 hour of active sonar pinging. COVIS ran this complete sequence once every three hours (8 times / day), with the minimum interval between sequences determined by the time required for packaging and compressing the sonar data for upload. A similar schedule of 8 acquisitions per day is planned for the OOI deployment. Under normal operation, COVIS follows a pre-programmed schedule performing sonar collections as defined in a configuration file, then packaging the resulting sonar data files for upload. After each sonar data collection, the control computer retrieves the sonar files from the sonar, packages them (see archive file description below), and stores them to an onboard disk for upload. Based on the data file sizes and schedule while deployed on Neptune, the control computer has storage space for at most one month of data collection; this permits the off-loading of data at a different schedule than the sonar operations schedule. COVIS Data Format Terminology: - MODE: One mode of sonar operation: plume imaging, plume doppler or diffuse flow. - PING: One sonar transmission. - SCAN: Set of pings are one elevation angle. - SWEEP: Complete sweep across a range of elevations, comprised of N scans (except for diffuse flow, where elevation is held fixed). - RUN: Complete data acquisition in one mode, comprised of 1 to N sweeps. - SEQUENCE: Complete set of runs spanning all modes. For each sonar run (one period of collection of one sonar mode), the data format as seen by the end user is a compressed archive (in the Unix-standard .tar.gz format) which contains the raw Reson IQ data files and metadata about rotator position during the sonar collection and other ping metadata. This archive is generated onboard COVIS by the SIC at the conclusion of the sweep and stored on disk in the SIC, where it can then be retrieved over the OOI network using standard network file transfer tools (e.g., rsync, scp, ftp, etc). As each sonar archive file corresponds to a single sonar mode, for the “standard” 8 sequences daily COVIS sampling routine used on Neptune, a total of 24 archive files are produced per day. As shown in Table 2, the archive file size varies greatly between sonar operating modes. Each archive file contains the following files 1. _sweep.json_, a json-formatted file that for each sweep describes start time, mode, and possibly additional annotations; 2. _transducer.xml_ gives a list of configuration values from the Reson control software. 3. _index.csv_, which contains a list of all the pings and the corresponding sonar orientations; 4. a number of record files (_rec_7038_xxxxxx.bin_ and _rec_7000_xxxxxx.json_) equal to the number of pings (where “xxxxxx” is the zero-padded ping number). The _rec_7038_xxxxxx.bin_ files are the binary sonar data (see Reson documentation), and the _rec_7000_xxxxxx.json_ files contain metadata describing the ping and the sonar. The number of pings varies depending on the mode, sonar operations (the setup is by time not ping due to specifics of Reson control commands), and other operations parameters. Each file is described in more detail below: sweep.json A sample sweep.json is shown below: { "mode": "imageleft", "_id": "COVIS-20181109T030002-imageleft", "starttime": [ 1541732527, 313958 ], "endtime": [ 1541733603, 73036 ], "motion": { "start": [ 25, 0, 70 ], "inc": [ 0, 0, 1 ], "steps": 105, "roundtrip": false, "mask": 2 }, "settings": { "rxgain": 53, "txpower": 220, "pulse_width": 0.0005, "range": 75 } } index.csv The index.csv file lists the ping number, Unix system time (in seconds and microseconds) for each ping, and the sonar attitude during the ping. The attitude is given in two triplets: - pitch, roll, yaw: - kPAngle,kRAngle,kHeading: The data is stored in CSV format with a single header line: ping,seconds,usecs,pitch,roll,yaw,kPAngle,kRAngle,kHeading 1,1541732528,233184,-1.0,25.1,70.6,7.4,-10.9,216.7 2,1541732528,688452,-1.0,25.1,70.6,7.4,-10.9,216.7 3,1541732529,188446,-1.0,25.1,70.6,7.4,-10.9,216.7 4,1541732529,688427,-1.0,25.1,70.6,7.4,-10.9,216.7 6,1541732534,887476,-1.0,25.1,71.0,7.4,-11.0,216.6 7,1541732535,387463,-1.0,25.1,71.0,7.4,-11.0,216.6 8,1541732535,887448,-1.0,25.1,71.0,7.4,-11.0,216.6 transducer.xml Transducer.xml is an XML configuration file generated by the Reson sonar. It is generally not required for post-processing and is not described here. sonar data files The sonar data itself is stored as pairs of files: - rec_7000_XXXXXX.json files contain a ping description, with XXXXXX being the ping number - rec_7038_XXXXXX.bin files contain binary Reson sonar data. A sample .json file contains: { "hdr": { "sonar_id": 0, "ping_num": 1, "multi_ping": 0, "xmit_freq": 396000, "sample_rate": 34482.758, "rcvr_bandwidth": 0, "pulse_width": 0.0005, "pulse_type": 0, "envelope_type": 0, "envelope_param": 0, "pulse_extra": 0, "max_ping_rate": 2, "ping_period": 1541732500, "range_sel": 75, "power_sel": 220, "gain_sel": 53, "control": { "auto_bd_filter_method": 0, "auto_gain_method": 0, "auto_range_method": 0, "bd_depth": false, "bd_range": false }, "prj": { "id": 1, "vert_angle": 0, "horiz_angle": 0, "vert_width": 0.0174533, "horiz_width": 2.0943952, "focal_point": 10000000, "window_type": 0, "window_param": 0 }, "transmit": { "pitch_stabilization": 0, "yaw_stabilization": 0 }, "hydrophone_id": 0, "recv": { "window_type": 0, "window_param": 0, "flags": 1, "beam_width": 0.008 }, "bd": { "min_range": 1, "max_range": 5, "min_depth": 1, "max_depth": 5 }, "absorption": 0, "sound_speed": 1468, "spreading_loss": 0 } }