Background and Terminology (4)
Note that acoustic waves in
the atmosphere travel with slower velocity than acoustic (and
other types of seismic waves) in the ground.
Atmospheric sound speed is temperature dependent, but
usually around 320-350 m/s for reasonable atmospheric conditions. Seismic velocities are as high as several
km/s in a volcanic edifice. For this reason
a conjoint seismo-acoustic source (i.e., radiating energy
simultaneously into the ground and atmosphere) will arrive
first on the seismic channel of a co-located seismometer and
microphone. The formula for expected
time delay of a conjoint seismo-acoustic source is:
Where D is the distance from
source to seismo-acoustic station, Vs and Va are the seismic
and acoustic velocities respectively and ΔTa-s is the measured delay time between seismic and acoustic arrivals.
