Seismic reflection

The return of an elastic wave from a contrast in material properties. At normal incidence in an isotropic medium, the conventional stress-amplitude reflection coefficient is $(Z_2-Z_1)/(Z_2+Z_1)$ for the relevant P wave or shear-wave impedance.

The ratio of reflected to incident wave amplitude under a stated physical-variable and polarity convention. For normal incidence between isotropic layers with impedances $Z_a,Z_b$, the stress-amplitude convention gives $R_{\sigma }=(Z_b-Z_a)/(Z_a+Z_b)$, while particle velocity gives $R_v=-R_{\sigma }$. The reflected energy fraction is $R_{\sigma }^2$ for a single lossless propagating mode. A receiver trace requires the relevant seismic wavelet, travel times and transmission factors as well as this local coefficient.

The inclination of a reflecting interface to horizontal. It influences seismic travel times, offset sampling and seismic migration; a reflector is not necessarily a fault plane.

The sign of a reflected seismic seismic wavelet relative to an incident or reference wave under a stated recording convention. A negative normal-incidence stress-amplitude reflection coefficient reverses that polarity.

A controlled-source survey recording reflected elastic waves to infer subsurface interfaces and their properties. Travel times constrain geometry through a velocity model; amplitudes also depend on source, propagation and impedance contrasts.

The number of source–receiver traces contributing to a midpoint bin or image location under a stated acquisition and processing scheme. Greater useful fold can improve the signal-to-noise ratio, but does not compensate for missing spatial bandwidth or an incorrect velocity model.

Processing that moves recorded reflection energy to its inferred subsurface positions using a velocity model. It corrects apparent positions of dipping reflectors and collapses suitable diffraction events; insufficient spatial sampling and incorrect velocities impair the result.

The ability to distinguish vertically adjacent reflectors. A quarter-wavelength thickness is a useful wavelet-tuning scale, rather than a universal exact limit; bandwidth, seismic wavelet shape and noise also matter.

Adjustment for the change in reflected travel time with source–receiver separation. A horizontal-reflector approximation gives $t(x{)}^2=t_0^2+x^2/V_{\mathrm{NMO}}^2$ under appropriate assumptions; a velocity model is required.