Methodology:

In December 2016, three seismic reflection profiles and a seismic refraction profile were acquired across Subglacial Lake CECs (Lines A/C/D). An additional reflection profile was acquired in January 2022 (Line B). Shot points were surveyed in at 240 m intervals using RTK GPS. Twenty metre deep holes were drilled with a hot water drill. Three 150 g Pentolite boosters, end to end and with two detonators (one detonator for line CECD), were lowered to the base of each hole and then backfilled with snow. Shooting was at least 24 hours after backfill.

Acquisition was carried out with two 24 channel Geometrics Geode systems using two 24 channel seismic cables. Four-element 40 Hz Georods were buried in-line with the cable to approximately 40 cm depth with the position measured by the cable takeout spacing. Shooting commenced at higher shot point numbers with the cables spread away towards higher shot point values. The spread had a 30 m offset to the first receiver and 10 m spacing between receivers. After each shot, the shot point was moved 240 m towards lower shot point numbers, one cable pulled past the other cable towards lower shot point numbers and the 24 georods redeployed. Four second record length with 8000 Hz sample rate.

In 2016 (Lines A/C/D), to ascertain surface elevation, all three reflection profiles were surveyed by driving a continuously recording dual frequency GPS unit along the profile (antenna mounted in a sledge at surface height). In 2022 (Line B), shot points locations were measured with a dual frequency GPS and receiver locations determined by interpolation (and corrected for 2 m survey pole height).

A seismic refraction profile was acquired along Profile A centred around CECA_0500 at W087.4529; S79.2439. The firn profile is assumed laterally homogeneous for this purpose and the shot points moved with receivers remaining in position from CECA_0030 to CECA_0500, except to acquire the closely spaced shots. Three shots were acquired to achieve a maximum offset of 930 m.

Data collection:

Seismic source: 450 g Pentolite in 20 m hot water drill hole, backfilled with snow.

Seismic source timing: GPS synchronised trigger.

Data logger: 2 x Geometrics Geode 24 ch 24 bit with GPS synchronised trigger.

Receivers: 4-element 40 Hz georod sensors on seismic cables with 30 m offset and 10 m spacing.

Data quality:

Due to a mis-fire, shot CECD_1440 was repeated with the new shot hole located 3 m offset from the original line towards the CECB line. The time interval between drilling to hole backfilling was only 45 minutes.

Similarly, shots CECB_1200 and CECB_1440 repeated as initial shot failed. Loaded and shot on the same day.

Seismic Reflection

Relative uncertainty in source locations is +/- 50 mm.

Relative uncertainty in receiver location is +/- 200 mm.

Absolute uncertainty in all positions is +/- 2 m.

Uncertainty in shot depth +/- 1 m but can be estimated from the surface ghost.

Seismic Refraction

Relative uncertainty in locations is +/- 25 mm to 50 m then +/- 200 mm onwards.