Methodology:

The acquisition instrument for this dataset was GLORIA II, operated on RRS Charles Darwin

The sonar towfish was 8m in length, and towed 300m behind the vessel at 50m depth. Data were collected on the RRS Charles Darwin CD37 scientific research cruise and stored on 9-track magnetic tapes, which are currently held by the British Antarctic Survey, Cambridge.

Raw data were recovered from 9-track magnetic tapes in 2021. Transcription from tape was performed by Troika International/Interica. There was some data loss, due to stiction causing read errors. Heat treatment of the tapes improved data recovery at this stage.

A Python package was developed by Paul Breen (BAS) to handle the GLORIA raw pass files.

Conversion to NetCDF and generation of quicklooks for the GLORIA data from CD37 was performed in 2024/2025 using the package. This can be accessed here: https://github.com/antarctica/bas-gloria

Decoding of the data has been performed following the structure as described in Table 1 of Searle et al. 1990.

The geographic position of the GLORIA instrument relative to the ship was calculated from the CD37 cruise track by introducing a fixed time offset of 90 seconds to the datetime of the ship at each recorded coordinate point. Latitude and longitude were subsequently interpolated, to increase the frequency of the coordinate information such that this could be added to the NetCDF attribute metadata by matching the track information with passes based on the datetime field.

NetCDF variable attributes:

- 'units': The units of the variable's data values. For this dataset, the value is "1".

- 'long_name': A long descriptive name for the variable. For this dataset, the value is "Scan Samples".

- 'description': Signal intensity of sidescan sonar expressed in counts.

NetCDF group attributes:

- 'pass_number': Pass number (units: 1)

- ''scan_number': Scan number (units: 1)

- 'hour_mark_flag': Flag for hour mark (units: 1)

- 'slant_range_correction_code': Slant-range correction code (units: 1)

- 'zero_flag': Binary zero (units: 1)

- 'pulse_repetition_period': Pulse repetition period (units: second)

- 'vehicle_heading': Vehicle heading (units: degree)

- 'year': Year (units: year)

- 'edge_mark_start': Unsigned 16 bit integer (units: 1)

- 'edge_mark_end': Unsigned 16 bit integer (units: 1)

- 'julian_day': Julian day (units: julian day)

- 'hours'''': Hours (units: hour)

- 'minutes': Minutes (units: minute)

- 'seconds'': Seconds (units: second)

- 'checksum': Checksum (units: 1)

- 'unused': Unused (units: 1)

- 'latitude': Latitude of GLORIA (unit: degree_east)

- 'longitude': Longitude of GLORIA (unit: degree_north)

- 'depth': Depth of the seabed from IBCSO at the location of GLORIA side scan sonar(unit: meters)

The cruise track CSV contains the fields:

- time

- longitude (degree_east)

- latitude (degree_north)

The GLORIA track contains the fields:

- time_gloria

- longitude (degree_east)

- latitude (degree_north)

- depth_IBCSO_v2 (m)

Data collection:

GLORIA (Geological Long-Range Inclined Asdic) is a side-scan sonar instrument which acquires acoustic images of the seafloor. Side-scan sonar allows to get an image of the seafloor through an active acoustic swath mapping method in which side-mounted transducers, suspended from a towed object, generate an acoustic signal which is reflected by the seabed and received by the transducer after a delay which is a function of the signal roundtrip travel time through the water column. The intensity of this returned signal enables the differentiation of seafloor features and can be processed into an image. This "image" represents how the sound produced by the sonar instrument interacts with the seabed interface.

As part of the CD37 cruise, the system consisted of two sonar transducers, each 5.3m in length and containing 60 elements over 2 rows. The transducers were inclined 20 degrees to vertical, directed to port and starboard respectively. The port transducer operated at 6.8kHz frequency and the starboard transducer at 6.3kHz frequency. The signal was a 100Hz swept frequency chirp, with a peak electric power of 5kW. The pulse frequency was one per 30s (a Pulse Repetition Period of 30-s), and pulse duration was 2 seconds. The range of GLORIA II is approximately 5 times the water depth, up to a maximum range of 22.5km on each side (i.e. port and starboard). This amounts to a total swath width of 45km.

GPS data was provided by a Trimble 4000 Locator and provides ship positioning information.

The software used to convert and process the GLORIA CD37 data are as follows:

bas-gloria package: https://github.com/antarctica/bas-gloria

Data quality:

In the initial data recovery, PASS245 was long and PASS246 was missed. This was determined to be caused by a reading error, resulting in concatenation of consecutive data from PASS246 to PASS245. These passes were then read from tape separately, and are now considered complete.

In the recovered raw data, some passes were missing: PASS039, PASS040 and PASS231. Copies of these data held at BODC were provided in CDF format, which were converted to NetCDF. These passes are now considered complete. As the CDF files did not contain data for the slant range correction code, zero flag, hour mark flag and unused attributes (which were determined to be non-significant in the recovered raw data), a NaN value of 9999 has been used in the converted files for these passes. In addition, it is to be noted that PASS038, PASS227 and PASS236 are partially complete,

To get the latitude and longitude of GLORIA related to the ship, a fixed 90 second time offset was added to the ship track datetime to account for tow distance of the GLORIA instrument behind the ship (the output of this was the GLORIA track CSV). The latitude and longitude of the GLORIA track was then interpolated to provide the value of these coordinates at the original datetime of the ship track. This was done in order to map geographic information to the timestamps within the netCDF metadata. Interpolation on the latitude and longitude fields from ship track data was done with the Pandas Python library.

Depth values within the NetCDF file metadata are provided by the International Bathymetric Chart of the Southern Ocean (IBCSO) version 2. Due to the resolution of IBCSO, the depth in the GLORIA NetCDF metadata is provided with a horizontal resolution of 500x500m, leading to some positive values close to coastline areas.