Converted-wave analysis

Abstract: Migration techniques, currently used in seismic exploration, are still scarcely applied in earthquake seismology due to the poor source knowledge and sparse, irregular acquisition geometries. At the crustal scale, classical seismological studies often perform inversions based on the arrival time of primary phases (P- and S-waves) but seldom exploit other information included in seismic records. Here we show how migration techniques can be adapted to earthquake seismology for converted wave analysis. As an example, we used data recorded by a dense local seismic network during the 2002 Molise aftershock sequence. In October and November 2002, two moderate magnitude earthquakes struck the Molise region (southern Italy), followed by an aftershock sequence lasting for about one month. Local earthquake tomography has provided earthquake hypocenter locations and three-dimensional models of P and S velocity fields. Strong secondary signals have been detected between first-arrivals of P- and S-waves and identified as SP transmitted waves. In order to analyse these waves, we apply a prestack depth migration scheme based on the Kirchhoff summation technique. Since source parameters are unknown, seismograms are equalized and only kinematic aspects of the migration process are considered. Converted wave traveltimes are calculated in the three-dimensional (3D) tomographic models using a finite-difference eikonal solver and back ray tracing. In the migrated images, the area of dominant energy conversion corresponds to a strong seismic horizon that we interpreted as the top of the Apulia Carbonate Platform and whose geometry and position at depth is consistent with current structural models from existing commercial seismic profiles, gravimetric and well data.

Abstract: The invention provides an acquisition method for a converted wave equivalent velocity ratio spectrum and a converted wave analysis method The equivalent velocity ratio spectrum acquisition method comprises the following steps: channels, of which in-phase axes have no change after going through a dynamic correction flattening process in a channel set in a converted spectral equivalent velocity ratio spectrum are rejected, and the remaining channels are taken as the channels participating in spectrum superposition calculation; and weighting coefficients of each time window is counted, and the amplitude value of each sample point in each time window is divided by the weighted coefficient of the corresponding time window, wherein the weighting coefficient is the average amplitude value or the maximum amplitude value after excluding an abnormal value of the corresponding time window; and the converted wave equivalent velocity ratio spectrum is displayed The converted wave velocity analysis method uses the equivalent velocity ratio picked up on the equivalent velocity ratio spectrum obtained by the above method to estimate a velocity field The equivalent speed ratio parameters extracted according to the method of the invention are more reliable and more accurate than estimated values or filled experience values, and the accuracy of the converted wave velocity analysis is high

Abstract: Accurate correlation of converted S-wave sections with P-wave sections has been one of the major challenges in the routine application of S-wave surveys. Careful study using borehole measurements can help resolve some of the uncertainties occurring in the converted wave analysis. This study uses conventional P-wave surface seismic, three-component VSP, and full waveform sonic measurements in the processing and interpretation P-Sv converted wave data from processing flow including wavefield separation, velocity analysis, and VSPCCP mapping Is used to construct converted Swave VSP sections. Correlation with P-wave data is good and some reflections are stronger and more pronounced on the S-wave section than on the P-wave section. There is also an indication of improved vertical resolution from the shorter wavelength Swaves.