Post-critical Reflections Research Articles

A ray parameter‐intercept time spectral ratio method for seismic reflectivity analysis

A method is proposed for obtaining reflectivity magnitudes from seismic record sections that are densely sampled in the horizontal dimension. The technique consists of finding the spectral ratio of subcritical (target) to postcritical (reference) reflection amplitudes. The method's novelty is that both target and reference amplitude spectra are estimated from slant (also known as ray parameter) intercept time, p‐tau or velocity) stacks of the record sections. The source level is directly determined because the reference amplitude is totally reflected; source level estimation does not require knowledge of the impedance structure of the reference reflector.The technique functions well for reflections having low S/N ratios, little or no lateralcontinuity, poor near‐surface velocity control and steep dips. Application of the method to the Consortium for Continental Reflection Profiling (COCORP) survey in the Rio Grande rift supports the interpretation that its large amplitude midcrustal reflection is a magma body.

Inversion of seismic data in the τ‐p plane

Seismic data can be transformed to the domain of intercept time and ray parameter without the artifical suppression of high frequencies caused by finite temporal and spatial sampling. These data can be analyzed to derive velocity‐depth information using the loci of the arrivals in the τ‐p plane. For limited source‐receiver offsets, the parameters of the best single ellipse approximating the true τ‐p reflection trajectory can be used to derive an approximate velocity‐depth function. We show that this is equivalent to Dix's (1955) hyperbolic traveltime approximation. When large aspect ratios are involved, the path formed by the postcritical reflection and refraction arrivals can be used directly to derive the velocity function. We consider both of these inversion methods as the initial step required to implement an inversion procedure that utilizes both pre‐ and postcritical reflection and refraction arrivals by an exact "normal moveout" correction in τ‐p. This approach is computationally efficient and is the equivalent to a ray traced normal moveout correction in X‐T.

Oceanic upper crustal structure from variable angle seismic reflection ? refraction profiles

The travel times of reflected and refracted arrivals observed on variable angle seismic profiles can be inverted to produce a first approximation to the variation of seismic velocity with depth. A method is discussed for deducing the interval velocity above a reflector by finding the best match between the observed variable angle reflection travel times and those derived by ray tracing through a series of trial models. Further constraints can be placed on the rate of change of velocity with depth by consideration of the amplitudes of both reflections and refractions. Synthetic seismograms calculated by the reflectivity method demonstrate that different kinds of velocity structure in the oceanic volcanic basement layer produce characteristically different variations of amplitude with range. In particular, the exact nature of the shear wave velocity transition at the top of the basement has a marked effect on the compressional wave reflection amplitudes. Pre-critical reflection amplitudes are seen to be generally more sensitive to the velocity structure at the sediment-basement interface than are post-critical reflections, whilst head wave amplitudes are strongly dependent on the velocity gradient in the upper part of the basement. The second part of the paper presents an analysis of variable angle seismic profiles obtained over the Madeira abyssal plain in the North Atlantic, using the travel time and amplitude interpretation techniques discussed earlier. It is concluded that the velocity structure of this area of typical oceanic crust comprises a stack of distinct layers within each of which the velocity increases smoothly with depth. An upper basement layer 600 to 700 m thick is a remnant of an originally lower velocity seismic layer 2A which is found on younger crust. Beneath this upper layer the compressional wave velocity increases with a gradient of less than 0.7/s to the base of seismic layer 2. The velocity gradient within the underlying seismic layer 3 is constrained by refraction amplitudes as less than 0.1/s.

Deep seismic zone as an upper mantle reflector of body waves

THE deep seismic zone associated with the Izu–Bonin arc dips at about 40° towards the south-west beneath the Dodaira Observatory, Japan (Fig. 1). At this observatory an unexplained phase with apparent velocity of 16.5 km s−1 was observed 10–20 s after the first arrival for earthquakes in the Ryukyu Islands. No corresponding phase has been found for earthquakes of different azimuths. This unexpected phase can be explained by post-critical reflection of P waves at the deep seismic zone after strong focusing by the 400-km transition zone: this zone of reflection is apparently sharp, of the order of 10 km in thickness. Typical ray diagrams are shown in the inset of Fig. 1. The ray path involves both the deep seismic zone and the 400 km transition zone.