Project Report 2019

Project Report 2019

Ongoing campaign in the caves of Orosei gulf: exploration, research, photogrammetry and 3D survey.

During summer 2019 more than fifty GUE divers gathered in the town of Cala Gonone, in western Sardinia. With GUE qualifications ranging from beginner cave divers to seasoned cave instructors, the team  had plenty of talent to put towards the project goals, the most ambitious and difficult of which were to capture 3D Photogrammetry of Bel Torrente cave and pushing the exploration in Cala Luna cave, continuing the survey, 3D modelling and cartography. Plus the ongoing scientific efforts regarding sediment/water sampling and survey of paleontological remains.

PHOTOGRAMMETRY:

The main objective was to capture 3D Photogrammetry of the entire Bel Torrente first sump, more than one kilometre. To make this challenging goal even more difficult, the Bel Torrente cave has a mixture of salt and fresh water, halocline, air spaces, and silt. The passage geometries also range from a couple of meters wide to twenty plus. And no-one has ever attempted to create such a 3D model before on this scale (Members of the team had done work in this cave and others previously and made smaller scale 3D models). The project concept started a year earlier with discussions between Andrea Marassich and John Kendall about the possibility of 3D mapping a cave. They rapidly brought Peter Brandt on board, and between them John and Peter came up with a plan for the 3D scanning. This then required designing and building appropriate rigs for scanning the cave.

In the months leading up the project Peter and John accumulated the necessary cameras and lights for the 360deg camera arrays. The first was made up of 12 cameras, the second of 8, and with both rigs utilising 8 wide angle video lights. To mount these onto the Suex XK1 DPVs required a whole lot of 3D printing, as some inventiveness. Unfortunately (as often is the case) the final build also required quite a lot of Gaffa tape and zip-ties. Each day ran pretty similarly, we had two boats, the Rebreather teams would leave first, and get into the cave first. They had the longest run times, and it was important for the Photogrammetry not to have other divers working the same areas. The OC divers would then follow 30mins or so behind. This worked well, and gave everyone space in the cave to achieve their goals.
At the end of the project more than 1TB of imagery had been created by the two photogrammetry teams, and processing had started, the cave was surveyed to the end of sump 2 in every passage, and everyone had fun and learnt stuff. Processing of the 3D data is still ongoing, but “early” results are looking promising.  

3D MODELLING:

During the project weeks it was planned to use state of the art survey techniques in close contact with the inventor of MNEMO and programmer of Ariane’s Line software to implement new routines and bring the software features to a new degree of development. Peter Gaertner spearheaded the efforts trying to create new in-water survey protocols with the development of technology available.

Key steps in the survey protocol were as follows: Defining a starting point of the survey in open water to be measured by GPS, installing markers on every tie off, ensuring a continuous guideline throughout the cave and – finally – the first step of our survey.

The basic line survey was done using the MNEMO. As a result the polygon representing the cave grew fast, and with this further tasks could be assigned: Measuring wall-to-wall und floor-to-ceiling distances (l-r-u-d coordinates) with tape measurements and taking photos. In order to get the proper shape of the cave, the strobe was fired between two plastic lids to get as a result a ring shaped flashlight, illuminating only the cross section of the cave, just at the correct angle in respect to the station.

The photos and the measures were processed into the software and together with Sebastian Kister, located in Mexico, a new feature was added: at every station of the polygon a photo could be added, together with the measured LRUD coordinates. The shape of the cave can be modelled by adding and moving anchor points to better represent the morphology.

A spline curve in the software allows to morph the shape of the cave in a much more accurate way than a classical diamond shape does; it is possible to add a number of anchor points to the curve, which allow to literally draw the cross section. Moreover, inclination of the plan on both vertical and horizontal axe can be adjusted, to better represent the cave and to show in the model specific spaces that would otherwise not be as clearly understood.

This allows for more accurate modelling of the cave and taking precise measures of the void area, so e.g. flow and water distribution can be modelled from scientists.

During the week tasks were assigned on a daily basis on the results of the day and the divers had the chance to change between different tasks to gather experience in deferent fields. As well the comfort level of the freshly graduated C2 divers could be stepwise increased, for which different parts of the cave gave a great opportunity. 

5529 m were surveyed, number of 772 stations, 8 major loops with error under 3%.

EXPLORATION:

The aquifer beneath the eastern Supramonte Plateau (which includes the caves Su Palu, Su Spiria, and Su Molente, which is connected to Bue Marino) discharges much of its water into the sea through multiple springs, with Cala Luna spring receiving a large volume of water during heavy rain and flooding. Dye trace experiments by local cavers have shown that inland cave systems connect to both the Bel Torrente cave system and Cala Luna, though these connections have not been confirmed by divers underground.

Unfortunately, despite sometimes presenting a strong flow that makes it impossible to dive, Cala Luna is characterized by increasingly smaller passages with complex navigation, multiple restrictions, and collapsed areas. As a result, explorers looking to connect to inland cave systems have failed to find the missing lead.

During an early dive in spring 2019 with Rob Neto, he quickly found a promising lead in a collapsed area towards the end of the mapped section. About 30 m (100 ft) further and in shallower water, at the edge of the collapse a restricted vertical crack in the middle of two solid, sheer, white slabs of rock stopped the exploration.

After multiple divers assessed the passage various times in the subsequent month, we realised the only feasible option was to push it with no-mount configuration. Considering the shallow depth, the crystal clear water, and the fact that the restriction was a simple and long corridor, we finally decided to go for it.

After the divers cleared the restriction they could gear up again in a relatively comfortable room, from which the cave continues west towards the inland systems, with the passages ultimately dictating the direction of exploration.

The diversity of this cave is amazing, as it changes from shallow, restricted, and silt-free to about 15 m (50 ft) wide at 30 m (100 ft) of depth; a mixture of sand and tiny gravel covers the bottom of the phreatic cross section. Big passages lead to rooms with apparently no ongoing passage, and smaller conduits led the teams inland and to open passages. This pattern repeated a few times throughout the cave. 

The team managed to explore and survey more than 1.4 km (0.9 miles) of underwater passages as well as some short, dry sections during six dives. 

There are multiple areas past the restriction where divers surfaced and checked the dry parts hoping to encounter other sumps. In some areas the teams checking the dry sections needed rope and single rope technique (SRT) caving gear.

As with many European caves, what poses the greatest challenge to the explorer is not the fact that there is a restriction but rather where the restriction rests, as it is many times a fair distance from the entrance; in this case, it is about 600 m (1,970 ft) from the entrance. This required a team of divers able and willing to carry safety cylinders, stages, DPVs, and extra stages and caving gear across the restriction.

Even though dives were as long as seven hours, shallow depths rendered decompression manageable. The main variable that needed careful management was coordination with the boat and surface crew, as they provided access to the cave. Weather and sea condition changes required efficient communication between divers and surface support. This resulted in a plan of a 6-hour dive time and the staging of two mobile phones in a predetermined location in the event of worsening weather and team separation.

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