DESCRIPTION:
Slope-Distance Measurements using EDM's and Theodolites

MSH89_outerflanks_monitoring_station_05-15-89.jpg
In addition to monitoring stations within Mount St. Helens' crater, the U.S. Geological Survey also maintained a network of monitoring stations around the base of the volcano. Angle and distance measurements to the volcano's flanks were taken periodically to watch for any deformation similar to the 1980 bulge. Pictured here is an instrument station northeast of the volcano.
USGS Photograph taken in May 15, 1989, by Lyn Topinka.
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Magma rising beneath a volcano forcefully displaces the surrounding rock, and the resulting deformation can be measured at the ground surface. Since about 1910, attempts have been made to measure this deformation in order to help understand magmatic processes and predict eruptions at active volcanoes. The vertical component of deformation has generally been determined by leveling techniques and by the less precise measurement of vertical angles. Early attempts to measure the horizontal component of deformation utilized relatively imprecise triangulation methods. In the mid-1960's, the development of the electronic distance meters (EDM) spurred personnel of the U.S. Geological Survey's Hawaiian Volcano Observatory to initiate studies of horizontal strain at Kilauea Volcano. These measurements have helped document repeated episodes of inflation and deflation of the shield volcano related to intrusions and eruptions.

Few such detailed and precise monitoring efforts have been attempted on active stratovolcanoes or composite volcanoes before 1980 (a notable exception is the study of Usu Volcano, Japan), when the reawakening of Mount St. Helens provided an ideal opportunity to test the utility of such measurements. In late April 1980, distance and angle measurements were initiated from instrument sites on the apron of the volcano to targets on its flanks. This monitoring documented a remarkable bulging of the north flank as magma intruded the volcano before the large landslide and explosion of May 18, 1980. The need to monitor all sides of a volcano was indicated by the localized development of the bulge, which covered 3-4 square kilometers, extended nearly 2 kilometers downslope from the summit area, and was mostly confined to a 60 degree radial sector of the cone. Points on the bulge moved tens of meters northward, whereas points just off the bulge and elsewhere on the cone were nearly immobile. After the May 18 events, geodetic monitoring of the volcano's flanks suggested slight horizontal expansion before other explosions in 1980 and slight contraction afterward. Since 1980, distance measurements inside the crater of Mount St. Helens have been used to predict dome-building extrusions of dacite a few days to 3 weeks in advance. The unprecedented success of horizontal strain monitoring at Mount St. helens suggests that this technique can be used for surveillance of other composite volcanoes, such as those of the Cascade Range.

Horizontal distance changes are measured using a variety of electronic distance measurement (EDM) instruments, which employ a laser beam aimed at a reflector station. The instrument compares the waveform of the light beam going out with that of the returning reflected beam and converts this difference to a slope distance. The magnitude and direction of the distance change are calculated for each station in the network. The displacements are relative to one or more points that are assumed to be fixed. The calculated displacement vectors are plotted on maps to locate the focus of deformation. The precision of the EDM data is +/- 6 mm, with an additional uncertainty of +/- 1 mm per km of line length.

Shortly after the May 18, 1980, eruption, geologists reestablished a surveying network on the volcano to measure changes that might signal additional eruptive activity. Glass prisms atop heavy steel towers on the volcano's upper flanks were surveyed with an electronic distance meter (EDM) and theodolite from instrument towers placed at the base of the volcano. ...

Our techniques in measuring ground deformation were based largely on experience at the Hawaiian Volcano Observatory with Kilauea and Mauna Loa Volcanoes. There, horizontal distances are measured by sighting to manned reflector targets with electronic distance-measuring equipment (EDM) ... and vertical changes are monitored by spirit leveling (Kinoshita and others, 1984). Modification of the Hawaiian approach was required at Mount St. Helens, because most of the desirable survey points were accessible only by helicopter, were frequently obscured by adverse weather, and were exposed to hazards from volcanic eruptions and from snow and rock avalanches triggered by the continuing seismic activity. In addition, the steep slopes of Mount St. Helens and the extensive snow cover prevented, without major effort and danger, establishing vertical control by leveling.

Accordingly, we began a program of measurements by theodolite ... and EDM, using unmanned permanently installed targets high on the volcano and three instrument stations at relatively low and stable locations ... The horizontal and vertical angles from theodolite observations, combined with a slope-distance measurements by EDM, yielded the unique location in space of the target relative to a single base station; repeated measurements permitted determination of 3-dimensional displacements. At the distances involved, about 2-4 km, the slope distances obtained are believed accurate to within +/-10mm; the uncertainties in the theodolite measurements, +/-5 s of arc, are about an order of magnitude greater.

The unmanned targets consisted of steel sign posts, on which were bolted wooden cross boards (the theodolite target) to which were attached inexpensive plastic highway reflectors 8cm in diameter (target for the EDM laser). ...

A slope-distance monitoring network on the flanks of Mount St. Helens was established one month before the major eruption of May 18, 1980. By 1990, the network has expanded to include 6 instrument and 13 reflector stations designed in a star-shaped configuration. The instrument and reflector stations are steel towers 3-4 m high cemented in the ground. The towers are designed to allow measurements to be taken during winter months when snow covers ground marks. For atmospheric corrections, measurements of temperature and pressure at the instrument station only (without corresponding measurements at the reflector station) have proved adequate for the precision required. ...

The purpose of this network is to detect a large influx of magma into the present conduit system. ... A star-shaped configuration was used to obtain coverage around the volcano ... Twenty-eight lines make up the outer net (Web note: "outer net" - name given to network around Mount St. Helens' flanks), with distances ranging from 2 to 8.5 km. ... electronic distance meter (EDM) to measure distances ... theodolite to measure angles ...

Trilateration and distance-measuring networks have been established on 12 potentially active volcanoes in the Pacific Coast States beginning in 1980. These networks, once the baseline information has been collected, can detect surface deformation that may reflect magma movement up the conduit. The rates of deformation increase as magma approaches the surface, and these measurements can therefore help determine where and when an eruption may occur. Before 1980, electronic distance meters (EDM's) had been used primarily to monitor horizontal deformation during inflation and deflation of shield volcanoes. Little horizontal deformation monitoring had been attempted on stratovolcanoes with the exception of Usu Volcano, Japan, where up to 160 m of movement was observed using trilateration techniques to monitor cryptodomes forming in the summit area in 1977-78. ...

In mid-April 1980, measurements of distances and angles were initiated at Mount St. Helens, primarily to monitor the rate of deformation of the bulge on the north side of the volcano. Displacements of 1.4-1.6 m/day were measured on the bulge prior to May 18, but there was little or no significant change outside the bulge area. A complete EDM network was established at Mount St. Helens shortly after the catastrophic eruption on May 18, 1980.

Monitoring networks were established at other volcanoes in the Pacific Coast States, including Augustine Island, Alaska ... Periodic reoccupation of these networks is planned as part of an overall long-term monitoring program. Reoccupation serves as a check on previous data, solidifies the baseline information, and provides an assessment of the state of the volcano.

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