Portable dense geophone array for shallow and very shallow 3D seismic reflection surveying - Part 2: 3D imaging tests

In part I of this paper we demonstrate how the use of a portable dense geophone array increases S/N ratio and the ability to record faint near-surface reflections. The dense array can also be used as an approximate acoustical lens with appropriate prestack imaging techniques, similar to those used in medical imaging and nondestructive testing. Such use of the array provides flexibility when imaging with sparse shot distributions and improves the overall resolution of the array. We show how, with prestack focusing, one can extend the subsurface coverage beyond conventional common-midpoint (CMP) coverage. We study optimal imaging configurations and show that although the effective aperture of the array is reduced when focusing off-center targets, we can still image small targets adequately. Maximum aperture and therefore best target definition occur when the source and geophone arrays are on top of the target. Both synthetic and field results show that the limited-aperture array can image very small targets with resolution of 0.25 m in three dimensions. We also demonstrate that different acquisition geometries should be used when imaging continuous geological targets versus small point diffractors.