In this section, we now seek to distinguish the geometric (specular) reflection contributions from the diffraction contributions to the total scattered response derived in (44). Without loss of generality, we define a local geometric/specular/Snell reflection wavefield such that
The term is a geometric reflection coefficient, which may or may not be related to the Zoeppritz plane-wave coefficient, as discussed later. The unit vector is the direction of geometric reflection given by satisfying Snell's Law at the reflecting surface, as sketched in Figure :
where is the unit normal to the reflecting surface at .Given this definition of , the following identity also relates to :
We derive the following useful properties for :
Then, we evaluate the stress field associated with this geometric reflectivity as:
Next, we decompose the total stress gradient into background and scattered stress field gradient components, as suggested by the Born analogy and physical interpretation of (36), (37) and (38):
where is the gradient with respect to a smooth (reflectionless) background model, refers to the direct stress divergence wavefield in the smooth background medium, and refers to the scattered (reflected) stress divergence wavefield in the smooth background model. There is an implicit and as yet unexplored relationship between the Born model (36) and our Kirchhoff model (51):
Our Kirchhoff decomposition (51) assumes a linear relationship between the total stress divergence in the true medium versus the direct and scattered stress divergences in the smooth background medium.
We evaluate the scattered stress divergence as:
The terms of order vanish because the gradient is taken with respect to the smooth reflectionless background medium, and contains no source of scattering. Continuing to evaluate :
where we have assumed that
which implies that that propagation directions , traveltimes and spreading amplitudes of the scattered wavefield as it propagates from the scattering point to the observation point, are approximately the same in the smooth background as in the true background medium. The divergence of the total stress field in the true medium is then:
The body force equivalent with respect to is:
Comparing (35) to (57) we see that
the volumetric body force equivalent geometric reflection coefficient can be identified as
Finally, we calculate the scattered field in terms of the geometric reflection coefficient using (39), (57), (45) and (40):
For clarity, we define the angles shown in Figure :
With these definitions, (61) can be written in compact form as:
To recap (66), the density at a subsurface point is denoted , and the geometric reflection coefficient at that point is . The amplitude terms As and Ar represent the cumulative geometric spreading, transmission loss, Q-attenuation, etc., from the source and receiver to the subsurface point respectively. The factor Ar also includes the vector component projection at the surface location as defined by (42) and (43). The term is the total traveltime from source at to the subsurface point and back up to the receiver at .Finally, the diffraction weight represents the angle between the anticipated geometric specular reflection direction and the actual diffraction direction . In the case of specular reflection when , and so .
We now make some general comments about the forward modeling theory given by (66).
This completes the forward modeling theory part of our paper that is necessary to proceed onward to the inverse estimation problem.