Field Update: Dec. 12, 2010

The next project on our agenda for this season is shot hole drilling – making small diameter (3”) holes penetrating about 35 meters into the ice and placing explosive charges in them that will be the energy source for a seismic refraction experiment. We surveyed out the area where the experiment will take place earlier in the season and placed flags marking each of the shot hole locations, which are 50 meters apart along a line going 2.5 kilometers to the west of the location of the proposed CH-1 drillsite on the seafloor (e.g., 50 shot holes). Each of the shot hole (energy source) locations are balanced by receiver locations that are spaced 25 meters apart along a line going 2.5 km to the east of the location of the proposed CH-1 drillsite (e.g., 100 receivers). During the experiment, each shot sends sound waves down through the ice, water, and sediment where the waves are reflected (bounce back) and refracted (change angle of travel) at each interface they encounter. We measure the time it takes for the waves to travel down and back (two-way travel time) and record the arrival times at each of the receivers (e.g., 100 source-to-receiver paths for each shot).

The shot holes are melted into the ice using a shot hole drill system that pumps hot water through a hose and nozzle to make the hole in the ice.

We melt snow in an insulated tank to provide the water supply for the drill. The water goes through a hose to the pumps and heaters on the next sled.

The heaters and pumps are connected by hoses to the water supply.

The hot water continues through a manifold to a hose reel mounted on a tower. The hose is connected to a nozzle to focus the water and melt the ice.

The nozzle is lowered into the ice as it melts downward to make the hole. One person controls the speed that the hose unwinds, while another guides the hose into the hole and makes sure that the nozzle melts a vertical hole.

Once the hole is completed, the explosives handler assembles a string of “boosters” connected by detonation cord and puts them into the ice hole.

The wire is tied to the flag to hold the charges in place while the hole seals off above them. The charges will freeze in place at the bottom of the hole and be ready to fire when the experiment begins in about a month.

Robin Bolsey, an expert in hot water drill systems for making shot holes explains the design of the system to Bob Detrick from the National Science Foundation (NSF). Each hole is drilled, the charge is emplaced, and then we move on to the next flag to repeat the process. A cardboard tube is placed around the string of 8 “boosters” to hold them together and make it easier to place them in the hole. This is another trick that we’ve learned by talking to experts. It takes about an hour to complete each hole when we first start this process, but as the team learns how to improve the sequencing and to balance the water level and temperature of the water as more snow is added to the tank, they hope to be able to reduce the time required to about 30 minutes per hole. The key is for each person to know their job and also know how their job affects the next step in the process. If we run out of water or the water is too cold, then the hole may not be melted deep enough into the ice or it may be too narrow to get the charge all the way to the bottom. If the hose is lowered too fast or too slowly, the hole may deviate from the vertical, or the diameter may increase in size. The shape, diameter and direction of the hole will determine how easily the string of explosives can be lowered into the hole from the surface to the bottom. If there are “bridges” or ledges in the ice due to changes in the way the hole was melted, then they may hang-up or get stuck and the hole will have to be re-melted.

After trying to place the charges in the second hole we melted, Ethan discovers that the detonation cord must be taped to the cardboard tube to prevent the “boosters” from slipping out of the tube during emplacement. Robin, Daren, Ethan and Graham discuss what to do and determine that melting a new hole is the only option since the cardboard was left down in the hole. Lesson learned we begin again – this time we’ll get it right.

The process of melting each hole uses up all of the water in the storage tank. This means that the snow must be continuously replaced in the tank by shoveling and the water must be heated and re-circulated to keep the temperature of the water in the tank above 70 degrees C to maintain the volume of liquid water by melting snow. Because the snow has a lot of air voids, it takes a lot more snow to make up an equivalent amount of liquid water, so Robin tries to keep the tank filled right to the brim. The heat capacity of water is also quite high, so by keeping the water above a certain temperature the snow can be melted quite quickly as long as the water level doesn’t decrease so much that the temperature drops. It’s a balancing act that requires constant attention. The heaters and the pumps can be adjusted to re-circulate hot water into the storage tank to help maintain this balance.

Robin knows that the heater exhaust can quickly dry his wet gloves. Clever!

The tower sled is positioned over the spot where the hole will be melted. The brake is released and the hose descends pulled by the weight of the nozzle.

Ethan has the charges ready in their cardboard tube, which is connected by wires to the surface. He is determined to successfully place the charges in the hole this time while he thinks of ways to improve the overall process.

The rest of the team looks down the line – only 48 more holes to drill!

Tamsin, Craig, Wallis and I return to Site #1 to check the mooring data.

The wind is starting to pick up in speed and although the sky is clear we expect the temperature to drop and the snow to blow around. The flags around the mooring site and the tripods are the only indication that we spent a few weeks at this site. The wind has shaped the snow into drifts that obscure any previous knowledge of our presence here. Craig connects his computer and downloads the data and then we depart in the Hagglund for Site #2 to check on the GPS station there – it has been sending its data back through the Iridium satellite but there’s been a bit of scatter in the data in recent days, so we want to check to make sure its still functioning correctly.

We drive the 10 kilometers back to Site #2, determine that everything is fine, and then set off for the ANDRILL camp at Site #3 while the wind continues to blow snow all around us. It will be nice to get a hot drink and a warm meal after this busy day for all. By the time we get back the wind has driven the gravity team back to camp and the shot hole group are on their way. Tomorrow will be another day and hopefully the weather will improve. Bob and I will be returning to McMurdo so he can continue on to South Pole for a look at what they do there. I’ll chase down some supplies and get the next group ready to fly to Coulman High in about a week to maintain our schedule. The rest of the team will continue melting shot holes, collecting gravity data, and running oceanographic instruments in and out of the hole.

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