Alistair J. Harding

A comparison of tau-rho transform methods

Many τ-p transform methods are available to seismic data analysts; selection of the appropriate method should depend on the nature of the source excitation, the intended use of the transformed data, limits imposed by sampling parameters, and computational cost. Using these criteria, we compare five methods on marine multichannel data and similar synthetic profiles. On fully sampled synthetic profiles, methods that handle the three‐dimensional (3-D) nature of the point source provide correct amplitude and phase information even at small slownesses, whereas 2-D and asymptotic approximate methods do not. When data from small ranges are not available, aliasing and truncation distort the amplitude of small slowness traces produced by all methods, but are most severe in the 3-D results. Transformation of data with increased input trace spacing or decreased depth to the reflector results in increased aliasing effects. When the intended use of the transformed data depends on correct arrival times only, and not on...

Numerical, Theoretical and Experimental Studies of Seafloor and Subseafloor Reverberation

Abstract : In studies of low-frequency reverberation within the marine environment, a central concern is the relationship between reverberation events and morphological features of the seafloor. A time-domain migration algorithm for the reverberation intensity field is developed that produces scattering coefficient maps coregistered with a bathymetry database. The algorithm is tailored to broadband transient sources with good range resolution, and was developed to analyze an extensive set of reverberation records from a 200-255 Hz source collected on the flanks of the Mid-Atlantic ridge. The precise, sample-by-sample, tracking of wavefronts across elements of the bathymetry database that forms the foundation of the algorithms implementation results in reverberation maps that show a clear and detailed correlation between scattering and morphology with narrow scarp slopes consistently highlighted. Environmentally induced asymmetries in transmission loss and incidence angle are exploited to break the inherent left-right ambiguity of the receiver array. Iterative migration, assuming a dominant dependence of backscatter on grazing angle, produces images, even from individual records, that show good ambiguity resolution. Results from multiple records corroborate the effectiveness of the ambiguity resolution and demonstrate the stability of the scattering coefficient estimates and the acoustic system.

On: “Suppression of sea-floor-scattered energy using a dip-moveout approach—Application to the mid-ocean ridge environment: discussion and reply

In their paper, Kent et al. (1996) present an excellent case history of the use of dip moveout (DMO) and velocity‐filtering in the common midpoint (CMP) domain for the suppression of out‐of‐plane arrivals scattered from a deep sea‐floor. However, they imply that as a result of a “small offset approximation” the use of DMO in this way is limited to surveys recorded in water depths of at least a few kilometers with conventional streamer offsets. This is incorrect. I argue here that the application of DMO will reduce to water velocity the stacking velocity of arrivals scattered from the upper surface of the seafloor without any restriction on water depth. Furthermore, I argue that this use of DMO is simply an example of the equivalence between 2-D and 3-D DMO for marine surveys where all source‐receiver azimuths are equal, and that no “small offset approximation” is required. I first present a counter‐example to the claim of Kent et al. (1996) that seafloor scattering cannot be suppressed using DMO in shallo...