Methods of tomographically inverting picked events on migrated seismic data

• US 20020183980A1
• Filed: 03/04/2002
• Published: 12/05/2002
• Est. Priority Date: 03/05/2001
• Status: Active Grant

• Current Assignee: Cggveritas Services SA (CGG)
• Original Assignee: Compagnie Generale De Geophysique
• Inventors: Guillaume, Patrice
• Granted Patent: US 6,577,955 B2
• Time in Patent Office: Days
• US Class Current: 702/189
• CPC Class Codes: G01V 1/303 for determining velocity profiles or travel times

Abstract

A method of inverting events that have been picked from a depth-migrated seismic trace, wherein a collection of traces is determined by depth migration prior to addition, the traces reflecting vertically below a given surface point, and processing is applied to said collection of traces in order to select a velocity field parameterization which optimizes the alignment of points which are migrated from said collection of traces.

Claims

1. / A method of updating a subsoil velocity model, in which:

depth migration prior to data addition is implemented on a set of seismic traces acquired in register with said subsoil, the migration being implemented with the help of at least one starting velocity model and serving to determine one or more trace collections each describing the subsoil vertically below a point on the surface;

one or more events which reflect vertically below the surface point under consideration are picked on at least one collection of traces obtained using said migration, and for each picked event a reflector depth and dip are determined, as is a reflector dip in register with said surface point; and

ray tracing is implemented between said reflector and the surface to determine collections of sound source and receiver pairs, together with data characteristic of travel times and time gradients which correspond to the ray traces associated with said pairs;

wherein subsequent processing is implemented in which the following steps are iterated;

dynamically migrating travel time and time gradient data previously obtained with the help of parameterization of the velocity field;

characterizing the alignment of the migrated points obtained in this way; and

updating the parameterization;

the processing selecting the velocity field parameterization which optimizes the alignment of said migrated points.

2. / A method according to claim 1, wherein the depth migration prior to addition of the seismic data is implemented for a plurality of velocity fields, and wherein, in order to determine the reflector depth and dip, that one of the velocity fields is selected which minimizes the departure from the horizontal of the depth/offset distance curve or of the depth/specular angle curve.

3. / A method according to claim 1, wherein the reflector depth is determined for different offsets or different specular angles.

4. / A method according to claim 1, wherein, in order to select a velocity field parameterization which optimizes the alignment of the points that are migrated from the time and time gradient seismic data, the distance is determined between the reflector portions seen for different offsets, with the selected velocity field parameterization being that which minimizes said distance.

5. / A method according to claim 1, wherein, in order to select a velocity field parameterization which optimizes the alignment of the points which are migrated from the time and time gradient data, the depth differences between the intercept points between a given vertical and previously updated reflector portions parallel to the reflector and which pass through the new migrated points, with the selected velocity field parameterization being that which minimizes said difference.

6. / A method according to claim 1, wherein in order to select a velocity field parameterization:

a local reflector passing through a migrated point is determined;

the other rays of the collection are traced between said local reflector and the surface;

a time error is determined which is a function of the difference between the travel time corresponding to at least one of said rays and the travel time of the trace which, in the collection, corresponds to the same offset distance or the same specular angle; and

a velocity field parameterization is selected which minimizes said time error and optimizes the alignment of the points which are migrated from the time and time gradient seismic data.

7. / A method according to claim 1, wherein, in order to select a velocity field parameterization:

a reflector is determined passing through a migrated point;

the other rays of the collection are traced between said reflector and the surface;

a time error is determined which is a function of the difference between the travel time gradient corresponding to at least one of said rays and the travel time gradient of the trace which, in the collection, corresponds to the same offset distance or to the same specular angle; and

a velocity field parameterization is selected which minimizes said time error and optimizes the alignment of points which are migrated from the time and time gradient seismic data.