A wavefront can be locally propagated by tracing local rays. Figure shows a cell in a triangular grid. At two corners of the triangular cell, the attributes of the wavefront are known to be
and
respectively. The wavefront attributes at the third corner,
can be determined through a procedure of propagating a local wavefront that passes the line boundary between the points a and b.
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Figure 4 Local wavefront propagation when the wavefront attributes at the points a and b are associated with a common wavefront. By tracing two local rays, the wavefront attributes at the point o is determined from the wavefront attributes at the points a and b jointly. |
The procedure begins with local ray-tracing from two corners of the triangular cell, where the attributes of wavefronts are known. As shown in Figure , the ray a shoots from the point a and in the direction of the gradient vector (u_{a},w_{a}), and the ray b shoots from the point b and in the direction of the gradient vector (u_{b},w_{b}). These two rays form a ray-tube that guides the propagation of the local wavefront passing between the points a and b. If a point on the local wavefront follows the wavefront propagation and reaches the third corner of the triangle, it must cross the boundary ab and propagate through the triangular cell. Therefore, the two local rays are traced as if the straight-line boundary ab extends out at both ends and the velocity field within the triangular cell extends out with the velocity gradient being unchanged. The wavefront attributes at any point on the rays can be calculated by using the formulas presented in the last section.
As shown in Figure , the local rays a and b are traced to the points and , respectively. At these two points, the normal distances from the third corner of the triangular cell to the rays are achieved. Let q_{a} and q_{b} denote the two normal distances, and define
Because the normal points and are on the rays, the attributes of the wavefront at these points can be computed analytically. Let be the curvature radius of the wavefront at the normal point .Then, the curvature center of the wavefront at the point is
(12) |
(13) |
The local wavefront within the ray tube formed by the rays a and b is considered to be continuous if one of the following two sets of conditions are satisfied:
When the local wavefront is continuous, the attributes at the points a and b are associated with a common wavefront. Therefore, the curvature center of the wavefront at the point o is estimated by averaging the curvature centers of the wavefront at the points and as follows:
(14) |
(15) |
(16) |
(17) |
The wavefront at the point o is approximated by a local circular wavefront of radius R_{o} and centered at (x_{o0},z_{o0}). As shown in Figure , the local circular wavefront at the point o intersects with the ray a at the point and the ray b at the point . Because these two points are on the rays, the attributes of wavefronts can be calculated analytically. Let and be the traveltimes at two intersections. Because both the point and the point are on the local circular wavefront defined at the point o, the weighted average of and is used to estimate the traveltime at the point o, that is
(18) |
(19) |
The last parameter to be estimated is the take-off angle of the ray that reaches the point o. It can be shown that, in two-dimension,
where n is the distance normal to the ray. Therefore, we have
(20) |
When the local wavefront is discontinuous, the attributes at the points a and b are associated with different wavefronts. Therefore the local wavefront at each point should be propagated individually. Figure shows an example in which the local wavefront at the point b is propagated. In this case, the curvature center of the wavefront at the point o is assumed to coincide with the curvature center of the wavefront at the point , that is, x_{o0}=x_{b0} and z_{o0}=z_{b0}. The curvature radius is then
(21) |
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Figure 7 The local wavefront propagation when the wavefront attributes at the points a and b are associated with different wavefronts. The wavefront attributes at the point o is determined from the wavefront attributes at each point (the point b as an example) separately. |
There are two special cases in which the rays a and b do not form a ray-tube because of multiple arrivals. Figure shows examples of two such cases. In both cases, the rays a and b converge with each other, that is
The first case has q_{a} < 0 and q_{b} < 0. The ray a guides the propagation of the local wavefront passing between the points a and b, whereas the ray b guides the propagation of another local wavefront. Therefore, only the ray a is used to compute the attributes of the wavefront at the point o. The second case has q_{a} > 0 and q_{b} > 0. The ray b, instead of the ray a, is used in the computation.
The wavefront may be diffracted at the points a and b because of the changes of the velocity gradients in the adjacent triangular cells. Figure shows an example of diffraction at the point b. Two rays that are traced with the same initial conditions at the point b split towards two sides. The local wavefront corresponding to diffraction is defined between two rays. The attributes of the diffracted wavefront are computed by tracing the local ray connecting the points b and o.
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Figure 9 An example of a diffracted local wavefront at the point b. |