Abstract:
The response of vertical arrays at single frequencies (CW) and for homogeneous media is well known. This paper addresses
the issues of frequency dependence and sound velocity gradients for the vertical array response in a deep ocean. I have
modified the synthetic seismogram code of Neil Frazer, Subhashis Mallick and Dennis Lindwall to address this problem.
The code uses a rearrangement of the Kennett reflectivity algorithm (Kennett, 1974, 1983) which computes the geoacoustic
response for depth dependent media and pulse sources by the wave number integration method. The generalized Filon
method is applied to the slowness integral for an additional increase in speed (Frazer and Gettrust, 1984; Filon, 1928). The
original code computes the response of a single source at a specified depth. The new code has several improvements over
the previous one. First, it is a much simplified code addressing only acoustic interaction. The total length is about half the
length of the original code. Secondly, the code can compute the response of a vertical array of point sources. By changing
the phase delay between the sources, we can steer the beam to the places of most interest. Thirdly, the code reduces
considerably numerical noise at large offsets. The original work has numerical noise beyond about 30 km offset at 50 Hz
which limits the application of reflectivity modeling in long range problems. The improvement comes with the optimization
of the program, both in the speed and program structure. The improved algorithm can be used to get the far offset response
(up to 150 km) of a vertical array in the deep ocean at frequencies up to at least 250 Hz. The modeling results are
compared to analytical and benchmark solutions. The modified reflectivity code can be applied to the study of
pulsed-vertical array sources such as were deployed on the ARSRP (Acoustic Reverberation Special Research Program)
acoustic cruises.
