High impact mass drops from helicopter: A new active seismic source method applied in an active volcanic setting

[1] We obtain estimates of the seismic velocity and attenuation for White Island volcano by use of high-impact sand-bag drops from helicopter. Three drops were attempted, two at either end of a 6-station linear array within the crater floor, and the third in the volcano's crater lake. The bags were dropped from ∼310–380 m height and contained ∼700 kg of sand. The impact velocity was estimated at ∼60–70 m/s yielding a kinetic energy of about 106 Nm, giving P-wave onsets to a distance of ∼1 km. We obtained a seismic velocity estimate ofVp = 1.2 km/s for the unconsolidated crater floor and Vp = 2.2 km/s for rays traversing through consolidated rock outside the crater. Attenuation was very strong (Q< 10) for both consolidated and unconsolidated parts of the volcano. This trial shows that low cost helicopter mass drops can be successfully applied to safely determine sub-surface properties at hazardous volcanoes.