Receiver notching in a linear v(z) near-surface medium
Dan Cieslewicz, Donald C. Lawton
Receiver notching occurs when a seismic reflection interferes destructively with its multiple off the surface. While common in marine seismic data, receiver notching does not occur in most land data, as geophones are usually placed at the surface where the interference is constructive. However, data from buried geophones, such as those from the Blackfoot III experiment, may be expected to contain receiver notches, which can adversely affect data quality. Receiver notching was investigated with computer simulations of a broadband Ricker wavelet traveling through a linear V(z) medium (as a first approximation of the true near surface) and interfering with itself after reflecting off a free surface. The effect of the interference on wavelet amplitude was also studied. Amplitudes were also investigated separately with an I(z) impedance model. The simulations indicate receiver notching occurs for depths of over one meter for near-surface velocity values typical of the Blackfoot III area. Interference decreases amplitudes to a minimum of about 27% of surface values, regardless of the velocity gradient. Below this depth, amplitudes increase to the value of the incident wavelet at a rate proportional to the average velocity of the media and the velocity gradient. The notches asymptotically approach zero frequency at greater depths at a rate inversely proportional to average velocity and velocity gradient. Additional high-frequency notches appear at depth and follow the same pattern, resulting in an often intricate pattern of notching at a function of depth. The impedance model amplitude simulations indicated that a medium of decreasing impedance would act as a natural signal amplifier, resulting in higher reflection amplitudes for shallower geophones. In several examples, the frequency spectra of reflections recorded by the Blackfoot III buried geophones contained notches not present in the surface geophone data. The converted-wave data contained more depth-dependent notches than the compressional-wave data, as predicted by the simulations. The amplitude simulations based on the impedance model strongly agreed with the real data, while the amplitude simulations from wavelet interference had little agreement. The study indicates that receiver notching occurs in buried geophones, and that the sharp impedance gradient of the near surface amplifies reflections, resulting in higher average absolute amplitudes for shallower geophones.