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| Titel |
Dropping sand bags from helicopters: A low cost and environmentally benign approach to determine subsurface velocity and attenuation structure of active volcanic systems |
| VerfasserIn |
A. D. Jolly, L. Chardot, S. Sherburn, J. Cole-Baker, B. J. Scott, N. Fournier, J. N. Neuberg |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250060198
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| Zusammenfassung |
| Obtaining estimates of the seismic velocity and attenuation structure of volcanic systems is
considered valuable from a monitoring perspective but can be extremely costly and time
consuming due to the potential environmental impacts, safety issues and the permitting
process. Here, we present an easy, low cost and environmentally benign alternative whereby
the shallow velocity and attenuation structure can be obtained via high impact sandbag drops
from helicopter. We conducted such a sandbag drop experiment at White Island
volcano on 23 September 2011, during the final stage of a 6 month deployment
of 14 broadband seismometers. Three drops were attempted, two at either end of
a 5 station linear array within the crater floor, and the third within the volcano’s
shallow active acid crater lake. The bags were dropped from ~400 m height and
contained ~700 kg of fine beach sand held within nylon sacks having a volume
capacity of ~2.0 m3. The impact velocity was estimated at ~70 m/s yielding a
kinetic energy of about 106 to 107 Nm. The source position was established by GPS
on the resulting impact crater and was accurate to within ~5 m. The lake drop
position was estimated from video footage relative to known ground features and
was accurate to ~30 m. Impact timing was achieved by drop placement close to,
but not on, the nearby seismometer recording systems. For the crater floor drops
the timing was constrained to within ~0.05 s based on distance from the closest
stations.
The low kinetic energy and strong attenuation of the crater floor meant that strong
first-P arrival times were limited to an area within ~1 km of the impact position. We
obtained a rough velocity estimate of about 1.0-1.5 km/s for the unconsolidated
crater floor and a velocity of ~1.5-2.0 km/s for rays traversing mostly through
the consolidated rocks comprising the crater walls. Attenuation was found to be
generally very strong (Q < 10) for both consolidated and unconsolidated parts of the
volcano.
Results show that low-cost sand bag drops can be viably used to determine shallow near
surface velocity and attenuation structure in volcanic environments where use of other active
source methods may be problematic due to environmental, permitting or cost issues. |
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