Surface and borehole geophysics

Ice-penetrating radar has made determination of ice thickness seem routine (e.g. Drewry, 1983b). The ability to map isochrons (Whillans, 1976; Vaughan et al., 1999b; Nereson et al., 2000) allows calibration of ice-flow models using tracer fields as described below (Clarke & Marshall, 2002), and assessment of basal melting (Fahnestock et al., 2001) and flow irregularities (Jacobel et al., 1996). Radar remains reasonably good at distinguishing frozen from thawed beds (e.g. Bentley et al., 1998; Gades et al., 2000), offers the possibility of assessing bed roughness (Doake et al., 2002), can contribute to study of c-axis fabrics (Matsuoka et al., 2003), and can even allow highly precise measurement of the time-rate of change of ice thickness or of basal melting (Corr et al., 2002). Improvements, especially in characterizing glacier beds, would be highly beneficial.

Numerous other geophysical techniques provide additional important insights (e.g. Shabtaie & Bentley, 1995; Behrendt et al., 1998). Of these, seismic studies are especially useful. Active seismic techniques, in which a signal is generated as part of the experiment (Figs 72.1-72.3), and passive techniques, using signals from natural earthquakes, are both of great value.

Active seismic techniques reveal much about the materials and conditions under ice, including both deep geology and the shallow conditions related to ice flow. For example, active seismic studies revealed the existence of soft till under West Antarctic ice streams (Blankenship et al., 1986; also see Anandakrishnan, 2003) and guided the borehole studies that demonstrated deformation (Kamb, 2001). The key role of geological control of those soft tills is also indicated by the co-location of ice streams over sedimentary basins (e.g. Anandakrishnan et al., 1998). Changes over time in basal properties can be monitored using active seismic techniques (Nolan & Echelmeyer, 1999). Important c-axis fabric boundaries detected in ice cores can be mapped across broad areas using seismic techniques (Fig. 72.4; e.g. Bentley, 1971).

Passive seismic surveys similarly produce insight to subglacial geological conditions through receiver-function analyses and identification of earthquake sources (Winberry & Anandakrishnan, 2003). Glaciologically, characterization of earthquake sources in and just under ice is especially relevant (Fig. 72.5; e.g. Anandakrishnan & Bentley, 1993; Anandakrishnan & Alley, 1994; Ekstrom et al., 2003). Moreover, the discovery of strong tidal control of Siple Coast, West Antarctica ice-stream motion was first made through observation of the timing of basal seismicity

Figure 72.1 Hot-water drilling for a seismic shothole, ice stream D, West Antarctica, 2002-2003 field season. (Photograph courtesy of Don Voigt.)
Figure 72.2 Don Voigt, Pennsylvania State University, and seismic blasting controller, ice stream D, West Antarctica, 2002-2003 field season. (Photograph courtesy of Don Voigt.)

of an ice stream (Anandakrishnan & Alley, 1997), and later confirmed and extended by GPS surveying (Anandakrishnan et al., 2003; Bindschadler et al., 2003).

The tidal oscillations of glaciers (Walters & Dunlap, 1987; O'Neel et al., 2001) and ice streams are quite interesting features (also see Kulessa et al., 2003). A highly successful investigative technique in numerous fields is to excite a complex system and then infer processes from the propagation and decay of the response. Specific hypotheses can be elaborated to produce predictions of the outcomes of such experiments, and the outcomes then used to discriminate among the hypotheses. Examples include pump tests or slug tests in hydrogeology, nuclear magnetic resonance (NMR) applications in materials science, high-pressure fluid injection triggering microearthquakes in seismology, and kicking the tyre of a used car to see if the door falls off.

Figure 72.3 Large seismic shot, ice stream D, West Antarctica, 2002-2003 field season. (Photograph courtesy of Don Voigt.)

Figure 72.4 Seismic section from West Antarctica. Left panel is from the West Antarctic ice sheet near the onset of streaming flow for ice stream C. A strong reflector from the bottom of the ice stream is visible near 1025 ms two-way travel-time, and a clear but weaker internal reflector is evident near 850 ms; we have interpreted this reflector in the ice as evidence of a change in crystal orientation fabric. The right panel is from farther downstream in ice stream C, and shows a strong basal reflector but less-distinct fabric development, perhaps because recrystallization has affected essentially the entire thickness of the ice stream (Voigt et al., 2003).

Figure 72.4 Seismic section from West Antarctica. Left panel is from the West Antarctic ice sheet near the onset of streaming flow for ice stream C. A strong reflector from the bottom of the ice stream is visible near 1025 ms two-way travel-time, and a clear but weaker internal reflector is evident near 850 ms; we have interpreted this reflector in the ice as evidence of a change in crystal orientation fabric. The right panel is from farther downstream in ice stream C, and shows a strong basal reflector but less-distinct fabric development, perhaps because recrystallization has affected essentially the entire thickness of the ice stream (Voigt et al., 2003).

Figure 72.5 Three-component seismogram from site E10 on the ice plain of Ice Stream E (top, vertical; middle, N-S; bottom, E-W). The clear P-wave arrival near 1 s and S-wave arrival near 1.5 s are from a basal event associated with motion of the ice stream.

Ordinarily, earth scientists are limited in their ability to excite a system, and thus must rely on unplanned events (such as high-pressure-injection 'disposal' of toxic wastes, or reservoir filling loading sensitive crustal regions) or natural sources (large earthquakes causing free oscillations of the planet). The tidal signal provides a beautiful excitation in glaciology; nature is kicking the tyres of tidewater glaciers and ice streams for us, allowing testing of hypotheses based on our knowledge and ideas about ice streams. Changes in the motion and seismicity of the ice reveal the response (Anandakrishnan et al., 2003; Bindschadler et al., 2003), and thus are targets for hypotheses.

The tidal data are among the many data sets addressing the basal boundary condition, the most important in controlling ice flow. Uncertainties about ice fabrics (Budd & Jacka, 1989) or temperature introduce errors of safely less than an order of magnitude in most ice-flow modelling, whereas uncertainties about basal conditions can bring order(s)-of-magnitude errors. Knowing more about glacier beds thus is central to our understanding. Borehole access is necessarily limited to restricted places, so surveys such as provided by seismic techniques are essential.

The techniques of seismic surveying are well-known and powerful (e.g. Jarvis & King, 1993). The difficulty remains of achieving extensive surveys—planting and powering geophones, drilling and using shotholes are neither easy nor fast. Numerous technologies have been tried, including 'streamers' containing geophones to avoid planting them (Anandakrishnan et al., 1998; Sen et al., 1998) and 'thumpers'to avoid shothole drilling. These innovations have had successes, but come with trade-offs in quality. Incremental improvements are expected to continue, but improvements comparable to the shift from pressure- to laser-altimetry are not expected at this time. Increased resources to allow more such experiments, and new ideas, will be required to really understand what is going on beneath the world's ice to control its flow.

Downhole surveying is another powerful technique for learning about ice. The possibility of seismic monitoring and shooting in boreholes, and the ability to use borehole seismic techniques to measure preferred orientations of ice crystals, recommend the technique (Thorsteinsson et al., 1999). Presently, many additional possibilities are being explored, including identification of annual layers and volcanic fallout and perhaps even location of biological materials (Price, 2000; Bay et al., 2001; Carsey et al., 2002, 2003; Hawley et al., 2003). It is clear that much more work is possible, with many additional 'payoffs'. Developmental work on the techniques, together with improved access through increased core drilling, borehole milling, or access-hole melting, may allow much progress quickly.

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