Mark Davies1, Hazel Rymer1 & Team Deformation, Montserrat Volcano Observatory2
1Volcano Geophysics Group, Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA
2Team Deformation 11/06/96 to 24/07/96. J.-C. Komorowski, J.B.Shepherd, G. Skerritt & R. Watts
Data compiled in a static gravity survey reflects the sub-surface geology and structure of a volcano. The information derived can be used to model the geological development of such structures and their interaction with, for example, the volcano plumbing system (Rymer, 1986). In a dynamic survey repeated high precision microgravity measurements provide valuable data on the sub-surface mass redistribution often associated with volcanic activity. The technique is a powerful tool in identifying precursory volcanic events (Rymer et. al., 1995), whilst during a prolonged volcanic eruption the data can be used to pre-empt a change in the current activity.
Gravity surveys using LaCoste & Romberg gravity meters G513 and D134 commenced in April 1996. The total gravity network on Montserrat now encompasses 84 stations. All points are marked with nails, whilst positions and elevations are fixed with the Leica 344 Global Positioning System and/or, the Leica TC1100 Total Station. 28 of the locations double as permanent microgravity stations. Currently the static gravity network is arranged in four radial lines covering the northern, eastern and western flanks of the volcano. All stations are referenced to the DOS point at M18, located on a rocky knoll at the eastern extremity of Harris town, 3.5 km from the active dome area.
The northern radial line extends from the GPS station at Farrells Road to the GPS station at Farrells Target and beyond, up to within 30m of the crater rim. The line comprises of 18 stations spaced 100m apart, except for points FT10 to FT15 where dense vegetation governs location. This line also incorporates the DOS point M58.
The eastern flank of the volcano encompasses the Tar River valley area which, at the present level of volcanic activity, is too dangerous to make gravity observations. Therefore, measurements were restricted to the Whites and Hermitage areas. 10 points spaced ca. 100m apart extend from the GPS site at Whites following a road up through Long Ground and then along a track to the permanent GPS site at Hermitage Estate.
Two radial lines have been installed on the western flank of the volcano. Running parallel with the road from the GPS site at Lower Amersham to the GPS site at Upper Amersham are 22 stations spaced between 50m and 75m apart. The line is further extended with 5 points along the footpath from Upper Amersham to the EDM site at Gages. The placement of these stations are governed by vegetation coverage and are therefore up to 150m apart. A second radial line is located further to the south, running parallel with the footpath at Chances Steps. 8 stations are located from the GPS site at the base of Chances Steps to the DOS point M9, located on Chances Peak. Again the spacing of stations is sporadic due to vegetation coverage.
No radial lines were set on the southern flank of the volcano due to time constraints. However, three gravity stations were established on Roches (Perches) mountain commencing at DOS point M4 and running parallel with the crater rim.
All data are currently being reduced to produce a gravity map and Nettleton density profiles. However, the map is incomplete as there are large areas of the volcano and surrounding flanks not covered. Therefore in December of this year four more radial lines together with numerous intermediate readings will be measured to produce a high spatial resolution (300m2) gravity map. All lines need to be extended further towards the crater rim and out to the coastline.
Between 30/06/96 and 24/07/96, repeated measurements were made at several of the microgravity stations. Whilst closure errors for each survey were less than 25Gals, the gravity readings at some stations fluctuated by as much as 80Gals. These variations occurred over periods of high rainfall and are therefore possibly reflecting a fluctuating piezometric surface. In December 1996 all microgravity stations will be re-measured and the results of the dynamic gravity over the last six months interpreted.
Oscillations observed on gravity meters:
Both meter G153 in April, and meter D134 in July detected "noise" in the form of long period oscillations. In all cases, the oscillations preceded an escalation in volcanic activity. This phenomenon has now been observed on three separate occasions in the form of a 4 second, and on one occasion, a 17 second oscillation. This effect has not been previously documented with gravity meters and is currently being analyzed on vibrating tables at the Open University.
Finally, dynamic gravity measurements are exceptionally important in monitoring the Soufriere Hills Volcano. EDM and GPS results do not show significant movement, whereas gravity appears to be continuously altering. This phenomenon has been observed on other volcanoes (Rymer & Brown, 1986). Hence, a volcano system without significant topographic deformation must have an ongoing gravity monitoring program so that variation in magma input, movement, vesiculation and density can be characterised, thus leading to a greater understanding of the volcano. A more long-term advantage of having a comprehensive microgravity monitoring program is that once the current eruption is over, the data can be used for predicting any subsequent volcanic crises.
Rymer, H. & Brown, G.C., (1986). Gravity Fields and their interpretation. J. Volcanol. Geotherm. Res., 27, 229-254.
Rymer, H., Cassidy, J., Locke, C.A., & Murray, J.B. (1995). Magma movements in Etna volcano associated with the major 1991-1995 lava eruption: evidence from gravity and deformation. Bull. Volc., 57, 451-461.