Head Wave Identification in an Offset Vertical Seismic Profile
, by Paul Fowler
In vertical seismic profiling (VSP), if the shot location is offset sufficiently far
from the well hole, converted and post-critical (head) waves may be generated and be
identified on the profile. An interface with high velocity contrast must be present
at a depth which is not too much greater than the offset if a head wave is to be
observed in the data. For a wide range of interface dips, if a head wave can exist,
it will always arrive earlier than the direct wave for the geophone at the head wave
interface. Becuase the head wave is an upgoing event, the curve delineated by the first
arrivals on a depth-time profile can show reversals in slope creating a local maximum
followed by a local minimum. Simple geometric modeling of travel times can provide
criteria for the presence or absence of head waves in offset VSP data and aid in their
recognition.
STANFORD