Seismic Refraction Studies on the Acoustic Basement in the Continental Shelf of the East Sea, Korea
Authors: Kim K.Y. , Kim H.J. , Han S.J. 
Published: October 2008

Critically refracted seismic waves can be utilized in elucidation of physical properties of a high-impedance acoustic basement where reflection methods are not effective in general. Quality factor Q was estimated for the high-impedance acoustic basement in the continental shelf of the East Sea Korea. Various pre-processing steps for the estimation included static correction for gun delay, geometric correction data interpolation and Fourier transform. Based on the constant Q model, quality factor was computed for the upper-most layer within the basement using the spectral-ratio method. The computation gives values between 10.4 and 13.6, which fall under the range for water-saturated clastic sedimentary rock.

Using seismic events refracted from the layer boundaries within the acoustic basement in the study are sedimentary layers were successfully defined. A short window AGC (automatic gain control) is an effective tool to reveal existence of the refraction boundaries. Based on a model for horizontal layers, refraction velocity and thickness were computed for each layer under a short seismic line of 3.75 km long in the continental shelf From four distinct linear events, three layers were interpreted under the water body of 150 m deep. The computed thickness is 275 m for the top layer and 200 m for the middle one. The corresponding refraction velocities are approximately 3900 m/s and 5100 m/s, respectively, The refraction event from the top of the lowest layer suggests that its phase velocity is greater than 6400 Ids.

Introduction

Lithology of the acoustic basement is often considered as an important factor by many industry geologists as well as academic scholars in order to elucidate geological history and formation of a sedimentary basin. For the critical angle at the top of the acoustic basement is generally very small, a conventional reflection method generally fails where the acoustic basement locates near the surface or just below the shallow water. Low primary-to-multiple ratios on reflection sections prevent from reliable geologic interpretation beneath the top surface of the basement.

As a part of a marine project commenced in 1994 (Ref.1), more than 310 km of reflection data comprising four seismic lines were obtained in the East Sea of Korea through 58 seismic charnels (Fig. 1). Stacked sections over the profiles show that several depositional sequences are well defined above the acoustic basement where the water depth is deep. On the contrary, primary reflections are not evident within the basement in the continental shelf. Thai is because seismic energy can not penetrate into the acoustic basement due to a large impedance contrast between the water layer and the basement. In this work physical properties, such as seismic velocity and quality factor, of the acoustic basement are computed from critically reflected events. Geologic structures within the basement are also to be investigated based on the horizontal layer rnodel Correlation to well data or previous Seismic data is not conducted since such data are not available.