The Seismic Wide Angle Reflection Method in the Study of Ocean Sediment Velocity Structure.

摘要

Although the wide angle reflection method was little used in the oceans, a well-developed mathematical theory and techniques for determining interval velocities within plane layers by this method existed for 20 years. The raypaths are least-time paths. Reflection curves from interfaces below the ocean floor are generally not true hyperbolas but are higher order functions forming curved lines on a T square-X square plot. Certain properties and limiting cases of reflection curves allow the computation of interval velocities both by exact and by highly useful approximate methods. These methods place stringent demands upon data quality, and to provide adequate data precision the ASPER (Airgun-Sonobuoy-Precision Echo Recorder) method was developed. The high data density provided by this method allows comparison of long segments of observational travel time curves with theoretical travel time curves appropriate for various physical/geological models. The method has several limitations; for example, it requires relatively simple, uniform geologic structure, and in deep water (2 km or more) velocities are only determined to about the nearest 0.1 km/sec, or even more if vertical intervals much smaller than 200 m are used. It is sometimes useful to find a regional velocity-depth function by combining the results of several sections determined in the area.