1887
Volume 36 Number 3
  • E-ISSN: 1365-2478

Abstract

ABSTRACT

The detection and resolution of a thin layer closely situated above a high‐impedance basement are predominantly determined by both the frequency content of the incident seismic wavelet and the existence of the nearby high‐impedance bedrock.

The separation of the thin layer and the basement arrivals is investigated depending on the low‐frequency content of the wavelet. The high‐frequency content of the wavelet is kept constant. The initial wavelet spectrum with low frequencies has a rectangular shape. All wavelets used have zero‐phase characteristics. Numerical and analogue seismic modelling techniques are used. The study is based on the geology of the Pachangchi Sandstone in West Taiwan.

Firstly the resolution of a thin layer between two half‐spaces is examined by applying the Ricker and De Voogd‐Den Rooijen criteria. The lack of low‐frequency components of the incident seismic wavelet reduces the shortest true two‐way traveltime by about 20%. In addition, low‐frequency components of the wavelet diminish the deviation between true and apparent two‐way traveltime by about 65% for layer thicknesses in the transition from a thick to a thin layer.

The second step deals with the influence of a high‐impedance basement just below a thin layer on the detection and resolution of that thin layer. Reflected signal energies and apparent two‐way traveltimes are considered. The reflected signal energy depends on the low‐frequency content of the incident wavelet, the layer's thickness and the distance between the basement and the layer. This applies only to layers with thicknesses less than or equal to one‐third of the mean wavelength in the layer, and a distance to basement in the range of one to one‐half of the mean wavelength in the rock material between layer and basement.

The minimum thin‐layer thickness resolvable decreases with increasing distance to the basement; i.e. for a layer thickness of one‐third of the mean wavelength in the layer the relative error of the two‐way traveltime increases from 5% to 30%, if the distance is reduced from one to one‐half of the mean wavelength in the material between the basement and the thin layer.

Finally, a combination of vertical seismic profiling and downward‐continuation techniques is presented as a preprocessing procedure to prepare realistic data for the detection and resolution investigation.

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2006-04-27
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