MVO/VSG - Open Scientific Meeting
27 November 1996

Methods of field survey of the dome growth at the
Soufriere Hills, Montserrat during 1996

Team Volume: J. Barclay1, S. Bower2, E.S. Calder1, P.D. Cole3, B. Derroux4, C. Harford1, R. Herd5, M. James6, C. Kilburn7, A.-M. Lejeune1, G.E. Norton5, J.B. Shepherd6, G. Skerrit4, R.S.J. Sparks1, M.V. Stasiuk6, N.F. Stevens8, G. Thompson9, J. Toothill6, G. Wadge8, R. Watts5 and S.R. Young5

1Geology Department, Wills Memorial Building, University of Bristol, Bristol BS8 1RJ
2School of Mathematics, University of Bristol, Bristol BS8 1TW
3Department of Geology, University of Luton, Park Square, Luton, Beds. LU1 3JU
4Montserrat Volcano Observatory, Old Towne, Montserrat, West Indies
5British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG
6Environmental Science Division, IEBS Bldg, University of Lancaster, Lancaster LA1 4YQ
7University College, University of London
8Environmental Systems Science Centre, University of Reading, Reading RG6 6AB
9Department of Earth Sciences, University of Leeds, Leeds LS2 9JT

Real time monitoring of the growth of a lava dome is important for several reasons. Regular measurements of the volume of lava extruded provide direct measurement of eruption rate. This provides constraints on the magma budget for the whole reservoir/conduit system. It also may have a direct bearing on the assessment of the likelihood of an explosive eruption. Measurement of the changing morphology of the dome provides information for local hazard assessment. A centre of dome growth moving to a new sector of the dome increases the hazard in that sector of the volcano downslope from the dome.

The growth of the Soufriere Hills dome has been monitored using several sources of data. In particular,two important techniques have been developed and employed at MVO since April 1996. Between April and July 1996 the dome was surveyed from a series of photographs obtained at three fixed locations around the volcano : Windy Hill water-tank, Harris Lookout and Whites. The separation of these locations is such that the photographs could be used in stereo-pairs to extract the three dimensional coordinates of identical points ( e.g. large blocks) identifiable on both photographs of the pair. Fixed reference points on the northern rim of English's Crater allow the geometry to be calibrated. From these sets of x,y,z values the dome surface can be estimated by interpolation (within the SURFER program) onto a regular grid. Comparison of grids with earlier topography enables incremental volume and shape estimates to be made. The extraction and computation of the points from the photographs, at first done manually, can now be done digitally. Photographs scanned as digital images can then have pairs of points picked by computer cursor and the geometry calculated semi-automatically to improve consistency. Sets of these photographs have been obtained regularly, weather permitting. Not all have been analysed yet but a substantial archive exists. One drawback with this technique is that it does not give information on the southern side of the dome.

In August a new technique was developed which complemented that using static photographs. This GPS-binocular technique allowed localised surveys at greater precision to be carried out and the southern side of the dome to be surveyed. The method involves tracking the exact location of a helicopter using kinematic GPS. One Leica GPS receiver on the helicopter is synchronised with a fixed receiver (at Whites) allowing post-flight processing to compute the position of the helicopter (to better than 1 m accuracy when hovering). From the hovering helicopter an observer views points (preferably vertically) to the surface of the dome with a pair of Leica Vector Infrared Laser Rangefinding binoculars which return distance (nearest m) and vertical and horizontal angle (to a precision of 1 degree). A second observer logs these values manually. From these data the x,y,z values of the points ranged on the surface can be calculated.

Although both these techniques are weather-dependent, less than comprehensive, and rely on trained staff (and in the case of the GPS-binocular technique, expensive equipment) for their execution they have both proved valuable monitoring tools in a situation where repeat photogrammetric survey is made impractical because of weather and logistics.

Montserrat Volcano Observatory