CHARACTERISTICS OF SECONDARY PG PHASES FROM OBS WIDE-ANGLE SEISMIC SURVEY AND THEIR ROLE IN CRUSTAL IMAGING

Abstract

The application of multiple waves is an important content of marine exploration, and eliminating or utilizing the multiple waves is one of the significant topics in the processing of seismic data. However, little work is concerned with the multiple waves of Ocean Bottom Seismometer (OBS) wide-angle seismic survey and taking advantage of them to improve the ability of seismic imaging. This study attempts to understand the characteristics of the secondary Pg phases and analyze the applications of seismic imaging in OBS wide angle seismic survey.

We firstly identify and know the secondary Pg phases from synthetic seismogram sections and record waveforms then calculate and analyze particle motions of primary Pg and secondary Pg phases through the azimuth angle rotation. After understanding the secondary Pg phases, we get the propagation path by the theoretical model simulation and calculation of measured data with the P wave travel forward modeling method based on the RAYINVR. In addition, improving the seismic imaging is expected, so we used the theoretical model and the actual model of OBS2010 to show the work of the crustal structure imaging.

The secondary Pg phases roughly parallel and follow closely the primary Pg phase, and are characterized by continuous, clear phase and strong amplitude. An obvious vibration is observed behind the vibration of the primary Pg with stronger amplitude, which is supposed to be the secondary Pg phase. On the basis of particle motions, the secondary Pg phases belong to the P-wave seismic phase. The travel-time fitting of the possible propagation path based on the test data gave three different results: (a) the χ² value is 14.921 when the reflecting layer is water layer and sediment; (b) the χ² value is 193.264 when the reflecting layer is the single water layer; and (c) the χ² value is 1.786 when the reflecting layer is the single sediment. After theoretical investigation and data tests, we have the following conclusions: (1) the secondary Pg phases are characterized by P-wave; (2) the secondary Pg phases are mainly from the reflection between the sediments, which (3) greatly increase the constraint on the basement, and (4) improve the imaging resolution of the sediments and the upper crust.