--> Joint inversion of first-arrivals with full waveform inversion: improving the near-surface model for depth imaging the Marcellus shale of Bradford County, PA
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AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Joint inversion of first-arrivals with Previous HitfullNext Hit Previous HitwaveformNext Hit inversion: improving the near-surface model for depth imaging the Marcellus shale of Bradford County, PA

Abstract

An important step in the drilling of horizontal wells for the exploitation of the Marcellus shale in Bradford County, PA is an accurate pre-stack depth image of 3D seismic data. Obtaining such an image requires overcoming several issues. First, the shale is underlain by the Syracuse Salt, whose movement leads to pillowing and faulting of the more brittle layers above it including the Marcellus. Second, the surface of the study area is very hilly, with elevations ranging from 600 to 1700 ft. Finally, and further complicating the picture, is varying thicknesses of glacial till, which are thin on the hilltops but a couple of hundred feet thick in the valleys. The velocities of unconsolidated glacial till can be as slow as 4500 ft/sec, leading to a strong disturbance in the depth image. The near-surface issues occur where seismic reflection data has a low fold and signal-to-noise issues which makes estimating velocity based on reflection moveout unreliable. Consequently alternative methods, not dependent on shallow reflections, must be considered. In our case study we investigate two methods, first arrival traveltime tomography and Previous HitfullNext Hit Previous HitwaveformTop inversion jointly with first arrival travel time tomography. The two methods are used to build an interval velocity model from the surface topography, which is then merged with an existing depth velocity model. Both models are used for prestack Kirchhoff depth migration and the resulting images are compared.