The Study On Velocity Field Of Bachu Uplift In The East Part Of Tarim Basin

Through analysis to the simple near surface model , it is known that the stackvelocity is smaller than the real one when we regard the flat smooth surface asprocessing datum, and bigger while taking the floor of flat smooth low velocityzoon as the processing datum . So the appropriate float datum should be between theflat smooth surface and the floor of flat smooth low velocity zoon.By analyzing simple subsurface forward models, it is presented that when thesubsurface layer medium changes gently , the stack velocity will change on thehorizontally , but it will not overturn in vertical;When the subsurface layer mediumchanges violently, the stack velocity gradient will be very great on horizontally andstack velocity overturn will occur to a more great extent vertically.Used two layer linear continuous medium model to calculate the averagevelocity along layers in Bachu area which shows: â‘  The average velocity of newerstrata above the Mesozoic Erathem is relatively small in the high part of a structureand high in the low part .â‘¡ Some older reflect interfaces may developed whoseaverage velocity is low in the high part of a structure and high in the low part on thearea which Cenozoic strata is relatively thick;When the Cenozoic strata is relativelythin ,the average velocity is high in the high part of a structure and low in the low part.â‘¢When the thickness of the older strata is to some extent, the average velocity underwhich will be high in the high part of the structure and low in the low part.According to forward modeling, horizontal velocity field variation will resultin the changes of structure trap area, the shift of the culmination and the structuredisappearance even pseudostructures. In conversion from time domain to depthdomain , the structure culmination shifts to the direction along which the velocityreduces. On the contrary , from depth domain to the time domain ,it shifts to the areawith high velocity. If the average velocity increasing direction is opposite to thestructure downdip, the structure will be amplified in in the conversion from timedomain to depth domain and pseudostructures occur. If the average velocityincreasing direction is in accordance with the structure downdip, the structure will bereduced in in the conversion from time domain to depth domain, resulting in thedisappearance of the structure.Utilizing smooth technology of frequency division and the two-step correctmethod, the stack velocity distortion and the closure were solved satisfactorily.With the multi-layer medium time-distance expressions, a new expressions isderived which is about the relationship between the interval velocity increment andstack velocity increment of the nth layer in horizontal multi-layer medium. From thenew expressions, a practical method of calculating interval velocity basing on modelwas put forward which has been proved a kind of practical technology with high inprecision through the real instances.