Gravity Methods
2.7 Applications and case histories
2.7.5 Volcanic hazards
Figure 2.58 Collapse in Reading city centre reveals the crown of underlying chalk mine workings located using micro-gravity surveys and follow-up probe drilling. From Emanuel (2002), by permission. [C]
particularly offshore where well control is sparse and observation wells are expensive. Advances in the accuracy of sea-floor time- lapse gravimetry, a ten-fold improvement between 1998 and 2006, have made this method feasible, as described by Stenvold et al.
(2008). Flooded areas can be quantified, providing complementary information to well-monitoring, production and 4D seismic data.
It is now considered feasible to monitor displacement of gas by water in reservoirs that are only a few metres thick. Gravimetric monitoring is now applicable for gas reservoirs of modest volumes (∼2×108m3in situ gas volume) at medium depths (∼2 km).
It is clear that routine sub-centimetre elevation measurements correspond to gravity changes of a few microgals, which is now within the resolution of modern gravimeters (typically≤5µGal) (Biegert et al., 2008). It is now possible to combine large-scale GPS and gravity surveys (i.e. hundreds of stations over hundreds of square kilometres) (Ferguson et al., 2008) with a similar precision being achievable with seafloor gravity measurements (Zumberge et al., 2008). Consequently many more applications of time-lapse (4D) gravimetric surveys, not only in the hydrocarbon industry, may be forthcoming in the future.
1998 2001 9
12
M GalM Gal
X X’
calc obs
calc obs
–10.0 10.0 30.0 50.0
Distance m
290.0 286.0 282.0 278.0 274.0 270.0 266.0 262.0
296.0 294.0 292.0 290.0 288.0 286.0 284.0 282.0 280.0 278.0 276.0
2.00
–2.00
–6.00 E
Depth
E
Depth
–10.00
–14.00
–18.00
–22.00
–26.00
–30.00 2.00
–2.00
–6.00 11 10
8 1
4
5
6 2 3
–10.00
–14.00
–18.00
–22.00
–26.00
–30.00
70.0 90.0
–10.0 10.0 30.0 50.0
Distance 70.0 m 90.0
2001
1998
Figure 2.59 Gravity models (2.5D) produced using GRAVMAG in 1998 and 2001 with the observed and calculated profiles attained in each year. The only interpreted difference between the two surveys is the upward migration of the low-density block (#9) towards the surface and the corresponding shallowing of the underlying material (#12). Adapted from Branston and Styles, (2003), by permission.
denser magma, can give rise to a rise in gravity, whereas mass loss through eruption or changes in composition of magma including foaming can cause the gravity to decrease. The elevation of the flanks of the volcano may change through inflation and deflation, causing a decrease or increase respectively in gravity. A study was undertaken by Currenti et al. (2007) of the 1993–1997 inflation phase on Mt Etna. They established a 3D finite element model
(FEM) in which they used the real topography (including geometry and seismic tomography) to infer crustal heterogeneities. Different geodetic surveying techniques clearly detected the inflation phase that showed a uniform expansion of the overall volcano edifice.
In order to obtain a reliable estimate of depth and density of an intrusion, joint modelling of the geodetic and gravity data should be undertaken. They showed that the application of FEMs allows
Table 2.9 Densities derived from an adjacent borehole and attributed to the polygons shown in Figure 2.59.
Polygon Density Polygon Density Polygon Density no. (Mg/m3) no. (Mg/m3) no. (Mg/m3)
1 2.65 5 2.69 9 2.44
2 2.70 6 2.20 10 2.45
3 2.30 7 2.65 11 2.2
4 2.65 8 2.72 12 2.55
for a more accurate modelling procedure and may help to reduce the misinterpretations of geophysical observations undertaken in volcanic areas.
There is a trend to install continuous gravity monitoring on volcanoes. This has the benefit of providing better environmen-
Residual Anomaly (R.A) values
-0.2 g/cm3
-0.3 g/cm3
-0.5 g/cm3
A-A
A A
A A
(A) (B)
Residential properties
Offices Car park
0 40 80
5
10
15
Depth (m)
GL
SECTION
Topsoil and misc. Fill.
Sand and gravel infill.
Soft clays and silty sands.
Glacial sands and gravels.
Bedrock.
Microgravity station Static cone test Borehole
0 10 m
Freemantle Terrace Residential properties
< -20 µGal -20 to -50 µGal
> -50 µGal
Residual anomaly
North Road
Figure 2.60 Micro-gravity survey (A) at Ripon, NE England, with the composite results of drilling and trenching along profile A-A’ on which the initial basic gravity model has been superimposed. S.R.=angle of shearing resistance assumed to be 50◦. Adapted from Patterson et al. (1995), by permission.
tal controls on the gravimeters used to restrict the thermal effects, and reduces the temporal aliasing arising from comparing grav- ity data acquired over different time sequences. For instance, if the acquisition of repeat data at a given station is too infrequent, discrete events within the volcano may be missed. Battaglia et al.
(2008) provide an example of a significant, but short-lived, event that was observed in continuous gravity monitoring during the 2002–2003 eruptive phase of Mt Etna (Figure 2.68). During tem- porary switches from vigorous lava fountains to mild Strombolian explosions, marked gravity decreases were observed to occur simul- taneously with tremor-amplitude increases (Carbone et al., 2006).
These changes in gravity were assumed to reflect collapses of the magma/gas mixture within the upper level of the system feeding the active vent. The collapsed column diminished gas flow to the shallowest levels of the discharge system to the atmosphere, creating conditions under which a foam layer forms. By substituting denser material (magma), a foam layer can induce a local mass (gravity) decrease. It can also radiate seismic energy by coupled oscillations
-0.593
Figure 2.61 (A) Residual gravity anomaly map over a prominent gravity ‘low’ in Ripon, NE England, with (B) the derived geological model indicating a dissolution breccia pipe and void at depth.
of the bubbles inside it. Growth of a foam layer could thus explain the observed joint gravity/tremor anomalies (Figure 2.68).
Montesinos et al. (2003) have described a detailed gravity inves- tigation undertaken over a volcanic crater in Terceira Island, part of the Azores archipelago. The island has been badly affected by earthquakes not only historically (e.g. in 1614 and 1841) but also in 1980, when the strongest earthquake to strike the Azores occurred (magnitude 7.2); other strong earthquakes have also occurred sub- sequently (1997, 1998). The majority of these earthquakes have been felt across the island but with higher intensity in the S˜ao Sabasti˜ao volcanic crater, in the southeastern corner of the island. The village of S˜ao Sabasti˜ao is located within this crater, which has an average diameter of 1100 m and a depth of about 50 m. Parts of this village have suffered more damage from earthquake activity than others. To
try to understand why, Montesinos et al. (2003) undertook a gravi- metric investigation, looking for a correlation among the geological, tectonic and volcanic characteristics of the area. The geology of the area is illustrated in Figure 2.69A, with the distribution of gravity stations shown in Figure 2.69B. The resulting Bouguer anomaly map of the area is shown in Figure 2.70, for which a terrain density of 2.48 Mg/m3has been used. The gravity map indicates an anomaly trending SSE from S˜ao Sabasti˜ao, which has been interpreted as in- dicating a volcanic and tectonic lineament. 3D regional modelling of the gravity data indicate a shallow mass deficiency in the cen- tre of the crater, which is shown in cross-section and plan view in Figure 2.71. The anomalous seismic response observed in the S˜ao Sabasti˜ao depression may be associated with a weak zone dominated by subsurface low-density structures, which are confined within
265 270 275 280 285 -0.04 -0.02 0.00 0.02 0.04
Elevation (m)g (mGal)
ρB= 1.6 Mg/m3
Original ground surface
Ice rich Unfrozen
Massive ice Approx. bottom
of subcut
Ice rich
Figure 2.62 Gravity profile across massive ground ice in a road cut, Engineer Creek, near Fairbanks, Alaska. After Kawasaki et al. (1983), by permission.
Walls
Crypt
Contour Interval 0.2 µm-2
0 10 m
-7.6 -7.2
-7.6
-7.2 -8.0
-7.6 -8.0
-7.6 -7.6
-7.6
-7.6
-7.6 -7.6-7.6
-7.2
-7.2 A
Figure 2.63 Micro-gravity map St ’Venceslas Church, Tova´cov, Czech Republic, showing marked anomalies over previously unknown crypts. From Blizkovsky (1979), by permission.
E mGal
W
-0.2 -0.1
-0.3 -0.4
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
measured modelled (II) modelled (0) modelled (I)
z [km]
-0.1 -0.2 -0.3 -0.4
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 1
2
6
x [km]
9 8
10
x [km]
5
3 2
7 8
11
3
11
10
5
8 4
3 4
Figure 2.64 Gravity profile across the Ellerbek valley, Germany, with the valley geometry taken from the interpretation of seismic reflection profiles. After Gabriel (2006), by permission.
[mGal]
0.6 0.4 0.2 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5
modelled measured
W E
z [km]
-0.04 -0.08 -0.12 -0.16
0.5 1.0 1.5 2.0 2.5 3.0 3.5 x [km]
Tertiary Quaternary Pleistocene
boulder clay
Figure 2.65 Gravity profile across the Trave valley, Germany, with the location of the valley based on a seismic reflection survey. After Gabriel (2006), by permission.
Observed gravity change Low temperature reservoir model High temperature reservoir model High temperature seismic model
6000 7000 5000
4000 3000
1000 2000 0
-20 -15 -10 -5 0 5 10
Gravity (µGal)
Smoothed time-lapse gravity compared with model predictions
Figure 2.66 Smoothed observed time-lapse gravity change plotted with modelled gravity change for high (average CO2density 550 kg/m3) and low reservoir temperatures (average CO2density 700 kg/m3) models. After Arts et al. (2008), by permission.
m 3000
Sea level
NNW SSE
+∆g
+∆P
-∆g
-∆P Fissure
+∆g
+∆P +∆g
(A)
(B)
(C)
0 5 km
Figure 2.67 Sketches of the stages of fissure eruptions on Mt Etna, Sicily, with gravity trends. After Sanderson et al. (1983) and Cosentino et al. (1989), by permission.
100 150
50 0
Velocity (µm/s)∆g (µGal)
–50 –100 –150 10 0 –10 –20
–30 19 Nov 2002
00:00
20 Nov 2002 00:00
21 Nov 2002 00:00
Lava fountain
Gravity station
Mild Strombolian
activity
Gravity station (local decrease)
Accumulation of a foam layer 07 Dec 2002
12:00
08 Dec 2002 12:00
09 Dec 2002 12:00
Figure 2.68 During the 2002–2003 Mount Etna eruption, gravity decreases lasting a few hours were observed simultaneously with increases in amplitude of the volcanic tremor. From Battaglia et al. (2008), by permission.
1 2 3 4 6 7
SERRA DO CUME
RIBEIRA SECA
0 0.5 1 km
N
Porto Novo
a b c d
Canada
de P onta 3
3
3 3
3
1
2
2
2
2 2
2
3 3
5
5 5
5 5
4
6
5 7
7 7
S. SEBASTIAO CRUZES
2 2
PICO DOS COMOS
PICO DO REFUGO CONTENDAS 5
5
5
5
2
491000 493000 495000
4278000 4280000
4282000
4279000 4280000
4278000
4277000
490000 491000 492000 493000 494000 495000
4281000 FONTE BASTARDO
(A) (B)
N
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.
mGal 140.5 140.0 139.5 139.0 138.5 138.0 137.5 137.0 136.5 N
Crater Coast line Sao Sebastiao
495000 494000
493000 492000
491000 490000
4277000 4278000 4279000 4280000 4281000 4282000
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).