Fresnel zones in the light of broadband data

Abstract

The investigation of zero‐offset response to circular reflectors of increasing Fresnel zone size shows that reflection response is a constant and is independent of reflector size, except when the reflector diameter is so small that the diffractions interfere with the primary reflection. The extent of this effect is dependent upon vertical resolution and the time separation of the primary reflector and the diffraction. Interference occurs for reflectors smaller in diameter than the first Fresnel zone. Migration removes this interference. For broadband data the Fresnel zone solution breaks into two parts: the primary reflector and the edge‐effects diffractor. With broadband seismic data, reflections and diffractions separate in time, except at locations near faults or very small bodies. Reflections are the seismic response to interlayer discontinuity and are independent of reflector size. Diffractions are the seismic response to lateral discontinuities and edges and depend on proximity to—and geometry of—the edge. Except in the locale of an edge, broadband reflections and diffractions are separated physically on the section and mentally by the interpreter. Furthermore, standard CMP processing attenuates diffractions, especially when CMP lateral offset is some distance from the diffractor.

Distribution of melt along the East Pacific Rise from 9°30′ to 10°N from an amplitude variation with angle of incidence (AVA) technique

“…In areas where the reflecting body is narrower than the diametre of the first Fresnel zone, the diffractions from the edges interfere with the primary reflection signal (e.g. Knapp 1991). In some sections of our study area an AML width of ∼500 m has been inferred (Kent et al 1990(Kent et al , 1993aCarton et al 2014), while elsewhere the width of the AML is on the scale of the Fresnel diametre.…”

Seismic attenuation tomography of the Tonga‐Fiji region using phase pair methods

“…Finally, the use of direct local P and S waves results in a small Fresnel zone, enabling high resolution with lower errors for the inversion. Assuming a frequency of 1.0 Hz, a 600 km deep event will have a Fresnel zone with radius roughly 50 km [Knapp, 1991 ], compared to a Fresnel zone of radius 160 km for teleseismic sS phases from deep earthquakes , or over 1000 km for teleseismic SS phases at their midpoint [Shearer, 1991 ].…”

Analysis of a seismic anomaly in Georges Bank basin, Atlantic continental margin

“…Careful velocity analyses in the anomalous zone assured us that we were attaining the optimum stacking response. Knapp (1991) showed how the seismic response behaves in a number of Fresnel zones for broadband data. From small targets, one can have decreases in the seismic response amplitudes at places where diffractions interfere destructively with primary reflections.…”