Study On Absorption And Attenuation Law And Compensation Of Low Retarding Zone In Thick Loess Region

The thick topography of the Loess Plateau is characterized by harsh terrain and complex surface structure.When the reflection wave of the target layer passes through the low velocity belt,the formation is equivalent to a low-pass filter.The energy and high-frequency components of the seismic wave are greatly absorbed and attenuated.This reduces the imaging accuracy of the mid-deep layers and structures,resulting in a reduction in the signal-to-noise ratio and resolution of the seismic data.The micro-logging data records the seismic waves passing through the low-speed belt and the multiple waves between the low-speed belts.The direct waves only pass through the low-velocity layer once,and can well reflect the energy of the seismic waves near the low-velocity zone.And frequency attenuation and change.Therefore,according to the micro-log data to invert the Q value of the low-reduction zone,and anti-Q filter compensation for seismic data can effectively improve the imaging quality of seismic data.Based on seismic wave absorption attenuation theory,combined with the characteristics of microwell logging technology,the paper applies three methods of VSP to estimate formation quality factor to microwell logging,and deduces the method of estimating the quality factor of microwell logging.The inversion method was validated by the forward simulation of viscoelastic medium in micrologging,and the inversion results of the optimal Q-value in the work area were determined by comparing the simulated data with the actual data inversion results.According to the selected Q-value inversion method,the Q value of all the micro well logs in the work area was inversed.The reversed filter was used to compensate the seismic superposition profile and the effects before and after the compensation were compared and analyzed.Concluded as follow:(1)The presence of high-frequency interference waves near the microwell logging borehole seismic track will affect the accuracy of velocity inversion and quality factor inversion,and average the common shot point gathers with the same well offset distance as the shot point record.Effectively mitigate the effects of random interference waves.(2)There are obvious differences in the structure of the low-velocity zone and the deceleration zone in the thick loess region.It is necessary to combine the quality inversion methods of the three microwell logs to estimate the Q value.In the shallow depth of the stratum,the micro logging well-ratio method and the centroid frequency deviation method can be used as the main method;in the deep part of the micro-logging wells,the micro logging well ratio method should be the main method,and the micro well logging center frequency deviation The amplitude ratio method is used as a reference for both methods.(3)The existence of volcanic voids on loess cliffs will lead to the development of vertical joints in the strata,making the closer to the cross-section the lower the velocity of the strata,and the correspondingly more severe the absorption of seismic waves;the statistics of the Q value of the well log and the longitudinal wave velocity Vp in the study area,The relationship between the quality factor and the P-wave velocity can be obtained for the low velocity reduction zone in the study area.For areas where the micro logging is not arranged in the study area or where the ground surface structure is similar,the low velocitydecreasing zone attenuation zone can be obtained from the velocity field based on the secondary relationship.(4)After the quality factors of the microwell logging inversion are applied to anti-Q filtering,the vertical resolution of the reflected waves in the coal seam is improved,the relative energy of high frequency components is enhanced,the effective bandwidth is widened,and the signal to noise ratio is improved.According to the quality factors of the microwell logging inversion,the characteristics of the attenuation in the low-velocity zone can be better reflected,and the anti-Q filter compensation method has a good application effect on nearsurface absorption and attenuation compensation.