3-D seismic data acquisition and processing - 3-D Seismic Interpretation - page 27

Receiver Boat
Line of Midpoints
Obstruction
Source Boat
Fig. 2.12 Principles of undershooting. Boats need to keep a safe distance from any obstacles and
therefore cannot acquire data close to rigs, etc. In order to record reflections from underneath such
obstacles, undershooting is used. This is achieved using two boats, a source boat and a receiver boat.
The source boat and receiver boat stay on either side of the obstacle with the source boat firing shots
to be recorded by the second boat. The reflection point lies halfway between source and receiver,
giving lines of reflected data from underneath the obstacle. It is still not possible to acquire some of
the short offset data, but the result is much better than having a no-data zone within the survey.
Since the source boat is not towing a large cable it is able to approach closer to the obstruction.
transmission of shear (S) waves. Recently (Garotta ( 1999 ) and Tatham & McCormack
(1991) ) there has been an interest in the use of marine shear waves, because they
have advantages in areas where gas clouds obscure deeper reflectivity and also be-
cause they may produce stronger reflections in areas of low P-wave acoustic con-
trasts. Another proposed use is in areas with very high velocity contrasts, such as in
imaging through salt or basalt layers; currently there are very few documented suc-
cesses. In such cases, the S-wave velocity in the high-velocity zone can be close to
the P-wave velocity in the surrounding sediments, giving a simpler ray-path for the
converted ray and the possibility of increasing the angular coverage at depth (fig. 2.13) .
Marine shear wave exploration requires conversion of the P-wave input to a shear
wave in the subsurface, normally at the target horizon, together with ocean-bottom
receivers to allow the reflected shear wave to be recorded (fig. 2.14) . Standard marine
acquisition contains some data that have converted from P to S and back to P again.
However, it is low in amplitude due to the double conversion, and the similarity in
terms of moveout to multiples means that its identification and separation is extremely
difficult.
The ocean-bottom receiver contains four phones, one pressure-sensitive hydrophone
as in normal marine seismic recording and three mutually perpendicular velocity-
sensitive geophones. This is often referred to as four-component recording, or 4C
for short. A variety of different recording configurations have been developed over