Gravity Methods
2.7 Applications and case histories
2.7.6 Glaciological applications
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Figure 2.69 (A) Volcanological map of the S˜ao Sabasti˜ao area of SE Terceira Island, part of the Azores Archipelago. (1) Old trachyte lava flows (Serra do Cume volcano); (2) old basaltic lava flows; (3) intermediate-age basaltic lavas; (4) recent basaltic lava flows; (5) basaltic scoria and lapilli deposits; (6) lahar; (7) fluvial and slope deposits; (a) scoria cone base; (b) crater rim; (d) volcanic and tectonic lineaments.
(B) Location of 344 gravimetric observation sites (dots) on a digital terrain model of the corresponding area. The highest point is 335 m.
From Montesinos et al. (2003), by permission. [C]
the crater and flanked by several denser bodies forming a resonant basin.
In a separate study, Camacho et al. (2007) have measured values of gravity at 253 stations across the volcanic island of Faial, part of the Azores archipelago, and lying on the Paial-Pico Fracture Zone which forms the current boundary between the Eurasian and African plates in the Azores area. The structural fabric can be seen in Figure 2.72A.
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Figure 2.70 Bouguer anomaly map corresponding to a terrain density value of 2.48 Mg/m3. Triangles indicate scoria cone eruptive centres. From Montesinos et al. (2003), by permission.
Their 3D inversion shows a subsurface anomalous structure for the island, the main feature being an elongated high-density body (Figure 2.72B and C). This forms a wall-like structure extending from a depth of 0.5 to 6 km b.s.l., and is thought to correspond to an early fissural volcanic episode controlled by regional tectonics.
Micro-gravity monitoring coupled with the distinctive patterns of the frequency of seismic activity (volcanic tremor spectra) pro- vide a very comprehensive model for volcanic eruptions, such as those at Mt Etna, and their associated processes. Many other vol- canoes now have active monitoring programmes utilising gravity, seismic and thermal investigations. Monitoring gas emissions is also proving to be a valuable additional indicator of impending volcanic activity (e.g. Pendick, 1995, on the work of S. Williams). If these data can be obtained for individual volcanoes in conjunction with thermal radiation as measured by satellite, then the probability of identifying recognisable precursors to eruptions may be enhanced significantly, leading to a better prediction of volcanic activity and mitigation of potential hazards (Rymer and Brown, 1986; Eggers, 1987).
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Figure 2.71 (A) Map of density contrasts at 0 m a.s.l. and the E–W profile for the line shown in (B) as derived from the gravity model.
The outline of the crater is shown by a dashed line; the web of lines indicate streets within S˜ao Sabasti˜ao. From Montesinos et al. (2003), by permission. [C]
observed and the bottom profile of the ice mass (i.e. the subglacial topography) can be computed.
Micro-gravity measurements have also formed part of a more comprehensive geophysical survey at Petrov Lake, Kyrgyzstan, as part of a glacial hazards investigation. While the quality of the grav- ity data was very good, the uncertainties in local density variations made it difficult to make specific interpretations about the pres- ence of stagnant glacier ice within a moraine dam. However, in conjunction with electrical resistivity tomography and self poten- tial measurements, the micro-gravity survey can provide additional contextual information that can help to constrain the overall geo- logical model.
2.7.6.1 Glacier thickness determinations
An example of this has been given by Grant and West (1965) for the Salmon Glacier in British Columbia (Figure 2.73), in which a gravity
survey was undertaken in order to ascertain the glacier’s basal profile prior to excavating a road tunnel beneath it. A residual gravity anomaly minimum of almost 40 mGal was observed across the glacier, within an accuracy of±2 mGal due to only rough estimates having been made for the terrain correction. An initial estimate of local rock densities was 2.6 Mg/m3, and the resultant depth profile across the glacier proved to be about 10% too deep (Figure 2.73B) compared with depths obtained by drilling. Considering the approximations taken in the calculations, the agreement was considered to be fair. In addition, it was found that the average density of the adjacent rocks was slightly lower (2.55 Mg/m3). This could have indicated that there was a significant thickness of low- density glacial sediments between the sole of the glacier and the bedrock. However, had more detailed modelling been undertaken on data corrected fully for terrain, the discrepancy could have been reduced.
Increasingly, ice thickness measurements are being made by seis- mic reflection surveying (see Chapter 5), electromagnetic VLF mea- surements (see Chapter 9, Section 9.6.6.3) and radio echo-sounding (see Chapter 9, Section 9.7.4.2). Comparisons between the various geophysical methods indicate that agreements of ice thickness to within 10% can be readily obtained. Gravity measurements over large ice sheets (e.g. Renner et al., 1985; Herrod and Garrett, 1986) have been considerably less accurate than standard surveys for three reasons:
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The largest errors were due to the imprecise determination of surface elevations; heights were determined to within 5–10 m;r
Inaccurate corrections for sub-ice topography vary by hundreds of metres, in areas without radio echo-sounding control. An error in estimated ice thickness/bedrock depth of 100 m can introduce an error of±74 g.u.r
As in all gravity surveys, the estimate of the Bouguer correction density is also of critical importance. Major ice sheets obscure the local rock in all but a few locations, and the sub-ice geology, and its associated densities, may vary significantly.In high mountain areas, the main disadvantage for the use of the gravity method, in addition to some of the issues raised for surveys over ice sheets, is the extreme topography and hence the need for detailed elevation information with which to apply ap- propriate terrain corrections. Seldom are such data available at the required resolution, so the inherent inaccuracies in the ter- rain correction are likely to be of the same order as some of the anomalies being sought. For example, in the Tien Shan moun- tains in Kyrgyzstan, a detailed micro-gravity survey was undertaken over a terminal moraine that dams a glacial lake in order to locate buried stagnant ice within the moraine. The uncertainty over the levels and accuracies of the necessary terrain corrections, as well as significant variations in local densities of the glacial sediments, makes the interpretation ambiguous. However, there is merit in developing this application if the control on the corrections can be improved.
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Figure 2.72 (A) Digital elevation model for Faial Island with the main volcano-tectonic structures superimposed. (1) Capelinhos Volcano;
(2) faults and (3) main volcano-tectonic lineaments. General volcano-stratigraphy: H: historical eruptions (with year of occurrence); C:
Capelo Volcanic Complex; V: Caldeira Volcano; B: Horta Basaltic Zone; G: Ribeirinha Volcano and Pedro Miguel Graben. (B) Example density model depth slice for 3 km depth, showing the high-density structure trending 110◦. (C) Density sections W–E and N–S along the profiles shown in (A). Adapted from Camacho et al. (2007), by permission.
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Figure 2.73 A residual gravity profile across Salmon Glacier, British Columbia, with the resulting ice thickness profile compared with that known from drilling. After Grant and West (1965), by permission.
2.7.6.2 Tidal oscillations of Antarctic ice shelves
Another glaciological application of the gravity method is the use of a gravimeter to measure oceanic tidal oscillations by the vertical movements of floating ice shelves in the Antarctic (Thiel et al., 1960; Stephenson, 1984). The resulting tidal oscillation pattern can be analysed into the various tidal components and hence relate the mechanical behaviour of the ice shelf to ocean/tidal processes.
If tidal oscillations are not found at particular locations, this may indicate that the ice shelf is grounded. Independent measurements with tilt meters, strain gauges and radio echo-sounding should be used to confirm such a conclusion (Stephenson and Doake, 1982).
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