Understanding the reverse time migration backscattering: noise or signal?

Abstract

Reverse time migration (RTM) backscattered events are produced by the cross-correlation between waves reflected from sharp interfaces (e.g. the top of salt bodies). Commonly, these events are seen as a drawback for the RTM method because they obstruct the image of the geologic structure. Many strategies have been developed to filter out the artifacts from the conventional image. However, these events contain information that can be used to analyze kinematic synchronization between source and receiver wavefields reconstructed in the subsurface. Numeric and theoretical analysis indicate the sensitivity of the backscattered energy to velocity accuracy: an accurate velocity model maximizes the backscattered artifacts. The analysis of RTM extended images can be used as a quality control tool and as input to velocity analysis designed to constrain salt models and sediment velocity.The analysis in this thesis suggest that we can use backscattering events along with reflection data to define a joint optimization problem for velocity model building. The gradient required for model optimization suffers from cross-talk, similar to the more conventional RTM images. In order to avoid the cross-talk, I use a directional filter based on Poynting vectors which preserves the components of the wavefield traveling in the same direction.Using backscattered waves for constraining the velocity in the sediment section requires defining the top of salt in advance, which implies a dynamic workflow for model building in salt environments where both sediment velocity and salt interface change iteratively during inversion.

Geometry Determination of a Foundation Slab Using the Ultrasonic Echo Technique and Geophysical Migration Methods

“…These events are usually considered noise and exist due to the correlation of waves which are not accounted for in the cross-correlation imaging condition [24]. In the left part of the migration result and in the area of the pile head these artifacts are clearly visible (dark shade).…”

Acoustic directional snapshot wavefield decomposition

“…Its biggest disadvantage is that the decomposition does not correspond to how acoustic data is generally acquired, which is generally on a single surface and not everywhere in space for one instance in time. This implies that this method is better suited for other applications, like decomposing wavefields as they are being modelled, for example using finite‐difference schemes, to improve reverse time migration (RTM) imaging results, see Díaz and Sava (). This is where the proposed approach excels as the wavefield is known everywhere from one time step to the next, allowing for directional wavefield decomposition at every time step, as opposed to having to record the wavefield and then later decompose it using conventional decomposition along some surface.…”

“…Wavefield decomposition is also an important tool in acoustic imaging, where it is either a pre‐requisite step before being able to image the subsurface (Wapenaar et al . ) or directly part of the imaging condition (Díaz and Sava ).…”

Adaptive variable-grid least-squares reverse-time migration

“…RTM has been widely studied and developed by many scholars because of its advantage in providing high-accuracy subsurface images (Sun and McMechan, 2001;Rocha et al, 2016;Du et al, 2017). However, RTM images usually suffer from artifacts (Zhang and Sun, 2009), incomplete illumination (Buur and Kühnel, 2008) and low-frequency noise (Díaz and Sava, 2016) because conventional RTM algorithm uses the adjoint of the linearized wave equation rather than its inverse (Nemeth et al, 1999). Inverse theory-based least-squares migration (LSM) (Lailly and Bednar, 1983) aims to obtain images with fewer artifacts and acquisition marks by approximating the exact inverse of the wave equation modeling operator (Lambaré et al, 1992;Kühl and Sacchi, 2003;Hu et al, 2016).…”