Small note on apparent sound speed and back azimuth
The above figure explains the terms apparent sound speed and back azimuth. The
left figure is a side view towards the array. A plane wave (blue) travels over
the array with the sound velocity. The mb's on the earth's surface measure the
horizontal fraction of the sound speed or apparent sound speed. The apparent
sound speed is a measure of the angle of incidence of the wave front. The apparent
sound speed equals the sound speed if the wave is traveling parallel to the
surface. The apparent sound speed is infinit in case the wave is coming from right
above the array.
The right figure shows a view from the array as if looking from above. The wavefront (blue)
travels over the array with a propagation direction given by the wavefront's normal vector
(thin black arrow perpendicular to the wavefront). The North is indicated by the thick
black arrow. The back azimuth is the bearing of the wavefront with respect to the North,
East is taken as 90 degrees.