Drilling and Deployment
Playing in the snow!
In IceCube, the optical modules span a depth from 1.4 kilometers to 2.4 kilometers below the surface of the ice. Getting that far down is no easy task. Several ideas were discussed from traditional drilling, to hot water drilling. In the case of traditional drilling, the cost of transporting the equipment was too high, and there's no guarantee that the hole will be straight. In IceCube the goal is to be as precise and exact as possible.
In the end, the method chosen was hot water drilling. This was the same method used in AMANDA deployment. Essentially you have a huge hose that blasts hot water out of its end, and by means of gravity melts its way down to the desired depth. Incredibly, after drilling almost 2.5 kilometers, the holes vary from vertical by less than a meter! Once the hold is drilled, teams have a short time to deploy the string carrying all of the DOMs before the water refreezes. Once the water is frozen, there is no way of retreiving the strings.
The drill being used in IceCube drilling is called the Enhanced Hot Water Drill (EHWD). This is an improvement over the AMANDA drill. The drill itself has a nozzle 2.5 inches in diameter, and when opertaing will require a flow of 10ft/sec. For the entire 2.5 km shaft, the drill will use 7000 gallons of fuel, and at a rate of 1.5 meters per minute will require 40 hours to complete its task. When drilling is complete, the result will be a cylinder 50 cm in diameter and 2410 meters deep.
For deployment, based on AMANDA experience, about 8 minutes was required per OM to deploy the string. If this remains true for IceCube, it will take a team of at least 7 people 8 hours to deploy a string with 60 OMs attached to it. For IceCube the time between deployments will have to be improved over AMANDA from 120 hours to 84 hours, running for 60 days, not including a 5 day down time estimate. This vigorous schedule will allow the all 16 strings per season to be deployed. Another time saving strategy is replacing simple wind shields with more stationary shelters for deployment teams. By deploying from an indoor facility, the workers can be more comfortable, and with a larger, more permanent type of facility, workers can house most if not all of the OMs, giving them access to the required equipment quickly. This graphic gives an estimate of the timeline for deployment of the strings.
As the strings are deployed, they will be able to be brought online within days of refreeze. As a matter of fact IceCube has been collecting data from the first season of deployment (austral summer 2005). This will not provide the most complete or accurate representation of the data, but it will be a good start, and as the rest of the strings are added, the abilities of IceCube will skyrocket.
Last year, the first IceCube string was deployed as a test, and the entire procedure went very smoothly. This graphic shows the timeline of the procedure, from when the hole was finished drilling and handed over to the deployment team, to when the line was tied off and work was completed. You can see that the original estimate of 18 hours per string was valid, and that deployment proceeded smoothly throughout the string. Where the line suddenly shoots up at about 23:00 is where there were no more OMs to attach. Remember that only the bottom kilometer is instrumented and after OM 60, the line can be quickly 'dropped' until the bottom OM lies at approximately at 2.4 km.
IceCube is still in the construction process, and each (Antarctic) summer there are a series of strings planned for deployment. This image shows the deployment plans for future strings. This will eventually contain all of the planned strings, but you can also see that these will not be completed until at least 2009.