For some time, environmentalists have worried about the breakup of ice shelves in the Antarctic. Do these changes stem from global warming, or do they simply occur from time to time during long, natural cycles? A careful study of the characteristics of the ice may yield clues about its long-term stability and movement. But such a study must observe ice in the same high-pressure low-temperature environment that exists in the ice flow. Only then can researchers obtain useful information about ice characteristics. Breaking the experiments into two smaller problems—environmental control and data acquisition—simplified the task.
Researchers in the Mineral, Ice, and Rock Physics Laboratory at the University College London (London, UK) developed equipment to reproduce conditions that exist in an ice flow. (Ice can exist at depths of 3 km, so it's very cold and under a lot of pressure.) An ice specimen—obtained as a drilling core—measures about 16 x 8 x 4 cm, and it sits in a vessel surrounded by silicone oil cooled to -40C and pressurized to 50 MPa. Internal hydraulic actuators, controlled in a servo loop, deform the ice along two axes. These actuators can apply a force up to 60 kN.
Gathering structural data involves placing a 3 × 4 matrix of transducers on a specimen across the two sides that don't have actuators pressing on them and by embedding transducers in the actuators' loading plates. These transducers transmit or receive pressure and shear waves through the ice. Earlier apparatus used stepper motors to independently position a transducer on each side of the ice, but environmental conditions and size limits in the pressure vessel made for unreliable motor operation, thus the change to a static array of transducers.
The PC-based instrument developed by the lab provides two separate functions. It controls the pressure vessel and the hydraulic actuators, and it controls the pressure- and shear-wave transducers and data-acquisition hardware. Multifunction 16-bit data-acquisition boards (National Instruments) and PID-loop software control the hydraulic system. Menus let users establish load and displacement settings for each experiment.
Divide and Conquer: Although linked by a
host PC, separate hardware controls the pressure and forces (red boxes)
applied to an ice sample and the acquisition of data (blue boxes) from
sensors on an ice sample's surface. Divising a complex task into smaller
pieces often yields a simpler, moremanageable test-system design.
How it works
During an experiment, a separate user interface lets researchers choose the transmitting and receiving transducers used for pressure- and shear-waves measurements. Instead of trying to force all the instruments onto PC add-in or instrumentation-bus cards, the system designers used a pulser/receiver from JSR Ultrasonics (Pittsford, NY) to pulse a selected transducer and obtain a signal from a receiving transducer. An external Agilent Infiniium digital storage oscilloscope (DSO) acquires the signal, digitizes it, and transfers the data to the computer under control of a LabVIEW program. Locking themselves to a specific "format" for instruments might have diminished the ability to get good data.
Each transducer can produce several hundred pulses per minute, from which the data-acquisition software computes velocities. These velocities, measured along multiple paths, let the software produce a velocity-distribution map from which researchers can infer how the ice sample's "fabric," or crystalline granularity, evolved. Information about the sample's fabric will reveal much about how ice flows and under what conditions the flow takes place. So the next time ice in the polar regions moves significantly, we may better understand why and how.