Port Miou

PORT MIOU NOVEMBER 2005

Water and sediment samples taken at -172m

2352m from the sea !

Photos (c) Jerome Meynie, Rose Yates, Mario Marconi, Many Armengaud 2003-2004-2005

(Click on any thumbnail to view full size)

The exploration of Port Miou Cave is an adventure between salt water from the sea (30g/L) trying to enter the cave through the tides of the Mediterranean sea and nearly non-salt water exiting from the cave.

The Romans knew the cave entry and came to collect the emerging nearly non-salt water. In the 70s, the Water Company of Marseilles (S.E.M.) built a dam (500m into the cave) to stop the sea water entering the cave during high tides, unfortunately upstream from the dam the water was still partially saline (10g/L).

Nowadays, the growing shortage of drinking water due to Global Warming once again raises questions such as is the water coming from far (2,3 km) and deep (-150m reached in the 90s) drinkable and/or useable ?

Ultimately, there is only one way to find out: we must obtain Samples as far and as deep as possible !

ROV ? too far (2,3km and going up down, changing directions ...)

Digging through the rocks on top of the big shaft at 2,3 km ? too deep, the shaft has a hill on top (minimum height 250m)

Sending a diver ? why not ; - )

You will need the best tools: Ouroboros CCR (Weapon of Mass Explorations), Silent Submersion scooters (shallow and deep versions), 4th Element Xerotherm Artic breathable base layer,
a light but robust dry suit DUI TLS 350

and my best friend on earth and underwater: Jean-Luc Armengaud !

All of the equipment needs to be very carefully balanced, both to go through the sea (250m, shallow -5 -3m depending of the swell, and against the current) and then through the cave where the salinity decreases.

To access the entry of the cave you must first take a little journey from the sea between boats and their anchors:

This is an example of some water exiting the cave, after a previous independent fluorescine coloration

The first sump (downstream sump from the Dam) varies in temperature between 13 celcius degrees, the entering sea water on the bottom of the sump (-13m down to the bottom -21m),

and 17 degrees, the top layer of water (-13m up to the ceiling at -6m in some parts). The aforementioned top layer of water is in fact the nearly non-salt water (freshwater) coming from sump two (upstream sump) and remains constant in its temperature at 17 degrees whatever the depth, -6m or -172m !!!

The difference of salinity between the two layers of water creates a halocline which forms a superb mirror, a wonderful and technical place for photographers !

Halocline in B&W (Photo by Mario Marconi May 2005 Expedition)

Entering the sump you have to scooter 500m (max -21m) to reach the 70's dam (Barrage).

Once you're there, the only way through is to go against the current via the tube-pipe (1m20 diameter) at -6m deep and for 5-6m long.

Meanwhile, on the surface (45m above the Dam) the old hut remains. Although now abandoned and dismantled the hut used to provide regular access to the shaft via a lift or stairs.

70's dam workers

If you want to go through the tubes of the Dam successfully, it is highly advised to do it at the end of a high sea tide.

There is up to 50cms variation between tides in the Mediterranean sea, many thanks to Henri Cosquer for that idea !

ATTENTION: Due to the research in progress by the Water Company of Marseilles (S.E.M.), only the big left tube-pipe is useable (it's equipped with a rope in it to pull on it if needed)

After the Dam the route contains 1,5 km of wide galleries, going up, down, and turning whilst the diameter varies from 8m to more than 22m (measured with the modified Echotest).

Maximum depth -40m but average around -25m.

Journey summary for the way in: Sea path 10 minutes; Sump one 15 minutes; Sump two 40 minutes (Silent Sub speed 9)

Semi-alpine style (enough for sanity breath and to come back on SCR mode) with S80s on the left Hogarthian style (but I still have my helmet!)

After 2 kilometers in the cave you reach "The Big Shaft" with his top balcony around -17m.

No decompression was needed on the way in (heliox 32/68 and flying PpO2 1,5b)

I descended in the Big shaft (where you don't see the opposite walls even with a 50w HID Metalsub) and quickly headed down (35m/min) until -120m (heliox 5/95, PpO2 1,45b).

At -120m I then started to probe for more depth with the modified Echotest II (filled with medical paraffin, like Mk15.5 HUD) whilst swimming in open water darkness away from the cliff.

After 2 minutes of probing at -120m, I had a +/- constant reading of 50 meters (so expected -120 -50 = -170m ?).

Before descending further, first I had to swicth ON the heating jacket.

This was mainly to avoid getting cold, due to the constant add of fresh heliox diluent by the automatic diluent valve of the rebreather, as muscle tremors can easily be confused with HPNS, .

After descending, I reached a roughly flat silty bottom at -172m, that confirmed the prior reading of the Echotest.

It took me less than 3 minutes to take some samples of water and sediment and to take a rough horizontal measure of the distance in front of me with the Echotest (> or = to 80m, max range of the Echotest).

At -172m the Ouroboros rebreather worked flawlessly, I used only the HUD leaving my hands free for my remaining task-load, I experienced no increase in the WOB (work of breathing) and a perfect scrubbing (patented design) of my CO2 in the loop.

Decompression on the way up went smoothly, I kept a good constant PpO2 of 1,55b

I had an unexpected encounter 2 km form the sea at -80m in the Big shaft with a little white shrimp, possibly a "Nyphargus spp" type.

As I still had two sample bottles left, I tried to capture it without success, it turned out to be much quicker and more agile than me ! Next time I'll use the proper scientific tool: a Bottle with two valves.

My last decompression stop in the Big Shaft was at -24m.

Having to go back down in sump two, I decided to shunt the -21m decompression stop

I went back into sump two on heliox 32/68 to save the cost of the heliox 5/95 and to avoid a too big drop of ppo2 when the ADV kicked in. I used a constant 1,5b of PpO2.

I was much quicker on the way back (30 min) than the way in (40 min) due to the current pushing me (low tide of the sea so less resistance against the outgoing freshwater current).

No more decompression stops were added, compared to what I had already on top of the big shaft, even with an up and down profile and a little distance to cover at -37m.

Before the Dam: I had a decompression stop at -12m at 1,6b PpO2. The calculated -15m stop disappeared on the last 200 meters of sump two.

I decided to shunt the -9m and -6m deco stops because the passage through the dam was at -6m and I could do them whilst exiting in the sea.

Going back through the tube (at low tide) of the Dam is a bit like going through a rocket launcher, you need to have all of your equipment (stage tanks, spare scooter) very near to your body.

It's probably the scariest part of the dive : )))))))

I went full speed through sump one (downstream sump) in less than 10 minutes at 1,6b of PpO2 with a maximum depth of -17m.

Decompression in the sea on the rebreather at -12m was on 1,6b PpO2, and at -9m on 1,9b PpO2 (diluent being pure O2; low ppo2 0,4b breaks every 25 minutes)

The sea was pleasantly warm (19-20 celcius degrees) having been heated through the day by the sun. I enjoyed the company of the fish and octopus coming to test the equipment, as they pulled on the hose of the breather !

The samples will help to understand the hydrogeologic system of the Port Miou Cave.

Some of the early conclusions of the analysis show that the water located at 2,3km in the cave at -172m is still 10g/L and contains some marine salt, therefore confirming the hypothesis cf Hydrogeology

Jerome and Jean-Luc: one complements the other

Many thanks to Jean-Luc and Many Armengaud for their help, friendship, and support without limits.

Many thinks to Thomas Cavalera who through the Water Company of Marseille analysed the samples. I'm also grateful for the time he spent answering my endless questions on hydrogeology, for providing all the B&W photos, topography and S8mm film.

Many thanks to Yves Billaud for the fantastic idea of the Echotest II

Many thanks to Kevin Gurr and all of his team of Closed Circuit Research Ltd (Jason, Tim �) for their work over the last 8 month on the fabulous CCR Ouroboros nb 002.

An old S8mm film exists about the construction of the Dam in the 70's it will be available in a ZIP format in the near future

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