Summary Undetected near-surface and subsurface velocity variations greater than an effective spread length produce false structures both in time and depth. The estimation of these variations rely on uphole measurements, high density shallow reflection data and sufficiently long offsets for subsurface reflectors. Despite inadequate near-surface velocity-depth measurements in the study area, we developed a strategy for the detection and estimation of both near-surface and subsurface long-wavelength velocity and low-relief structural variations. We chose two 2D seismic lines (dip and strike) over two existing low-relief fields in central Saudi Arabia, to demonstrate how the velocity-depth ambiguity problem is partially overcome by integrating the refraction delay time solutions derived from multiple refractors below the seismic reference datum with iterative pre-stack depth migration, residual analysis, and reflection tomography. We compare these results with the conventional isopach depthing method and discuss how this strategy improves the structural integrity of potential prospects. Even with the verification of the near-surface velocity and structural anomalies along the dip line and buried velocity anomaly along the strike line through forward modeling and flatness of depth image gathers, these models are non-unique. Based upon this case study, a new constrained tomographic algorithm has been developed which overcomes the above ambiguity and is discussed in Part II. Introduction In central Saudi Arabia, 2D seismic data is acquired along lines (greater than 70 km) over near-surface conditions which include different combinations of sand lenses, buried channels, leaching, outcropping formations, and in some areas the effects of a shallow unconfined aquifer. These variable near-surface conditions give rise to lateral velocity variations and thus to time anomalies of wavelengths of different scales, from as small as a cdp interval to much greater than a cable length. The base of weathering for short wavelength velocity variations is confined to several tens of meters below the surface and extends well below the seismic reference datum (SRD) for long wavelength lateral velocity variations. In the study area, the sparsely spaced and shallow uphole control points could not be used to generate a reliable 3D average velocity model because the base of weathering is well below the maximum uphole penetration depth and SRD
