Figure 77.1 Sketch of the borehole tools. Up to three probes can be placed inside the drill rod. More would interfere with the couplings between the 3 m rod sections. After hammering the probes are pulled out by the detachable drive point that acts as anchor. The hammer (shown in extended position) is connected to the drill rod via an adapter. A variable amount of weight is added to increase the active hammering weight. The hammer is accentuated with a composition rope.
whether a light rope or cable can be used to actuate the hammer from the surface. We decided on a 12 mm diameter rope after a full dynamical calculation of the effects of its stretch on hammer operation. Part of the problem is to determine the position of the hammer with respect to its anvils, of which there are two to enable it to hammer either up or down. The surface equipment consists of a tower to assemble down-hole equipment, a heavy tripod to guide the rope down the hole, and a 37kW engine driving either a drum containing the rope when raising and lowering, or a cathead (a rotating friction drum) to engage the rope (or a short piece of natural fibre rope attached to it) when hammering. The total weight is 2.3t.
The instrumented probes are placed in a section of 67 mm diameter drill rod (9.5 mm wall thickness, Fig. 77.1), after being tied to each other and to a detachable drive point, all of which is driven into the till by the hammer. Upon reaching the maximum depth, the action of the hammer is reversed, and the drill rod hammered out. The drive point has a larger diameter than the drill rod and remains behind, pulling the probes out of the drill rod as it is retracted. The maximum till depth reached was about 2.5m.
77.3 The probes
Our probes were shock mounted to survive damage or calibration loss during hammering (Fig. 77.2). They measured pressure
Figure 77.2 Interior of probe, consisting from left to right of transmitting coil, four lithium cells, electronics and tilt sensors, and pressure transducer. This assembly is then cast in epoxy, wrapped in styrofoam and packed into a casing. The casing is 610 mm long and 44 mm in diameter. The wires will be cut after the probe is programmed.
and two axes of tilt, which were sent, without wires, to a receiver just above the ice-till interface using a low-frequency magnetic field. We would have used a relatively high-frequency electromagnetic field, which might have avoided the need for a down-hole receiver, except for the difficulty of transmitting through the heavy metal cases, which we thought might be necessary for survival of the probes in active till. The question of whether plastic casings would survive in such conditions remains open. At any rate, the wireless capability avoided the difficulty of operating a heavy moving hammer in the presence of signal cables, and was essential in our judgement. The probes were designed to transmit data once a day for a year. Data reception was intermittent owing to relatively straightforward electronic problems that could not be fixed in our single season of testing (beginning in April 2002). More details are given by Harrison et al. (2004).
This work showed that a heavy down-hole hammer can be operated from the surface with a long light rope, and that wireless communication from instrumented till probes is feasible. Without a drill rig, a heavy hammer seems at present to be the best tool for inserting instruments into till.
Nevertheless, penetration of clast-rich till is still a problem. We penetrated only a about a third of it, although more progress could probably have been made with more weight on the hammer. A more subtle and perhaps ultimately more difficult problem is to determine the nature of the ice-bed interface ('diffuse' or 'sharp' in the simplest terms), and exactly where the probes are located with respect to it. This is a problem first because hot-water drills can penetrate sediment, and second because loose material sometimes intrudes into a borehole, as is well known in commercial drilling. The mechanical problems of penetration and location of the interface are more difficult than the more glamorous one of wireless communication.
Remembering that till is not a continuum, the ultimate unanswered question is what types of sensors should be in the probes, and what the dimensions of the probes and their separation should be. An iterative process probably will be needed. As deformation mechanisms become better understood, the instrumentation can be optimized to study them.
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