Method for using mode converted P- to S- wave data to delineate an anomalous geologic structure

• US 5,200,928 A
• Filed: 11/07/1991
• Issued: 04/06/1993
• Est. Priority Date: 11/07/1991
• Status: Expired due to Fees

• Current Assignee: Chevron Research And Technology Company (Chevron Corp.)
• Original Assignee: Chevron Research And Technology Company (Chevron Corp.)
• Inventors: MaCleod, Mark K.
• Primary Examiner(s): STEINBERGER, BRIAN
• Application Number:
  US07/789,229
• Time in Patent Office: 516 Days
• Field of Search: 181/102, 181/112, 364/421, 364/422, 367/25, 367/27, 367/33, 367/40, 367/53, 367/72, 367/73, 367/75, 367/57
• US Class Current: 367/27
• CPC Class Codes: G01V 1/284 Application of the shear wave component and/or several components of the seismic signal

Abstract

A method for using mode converted P- or S- wave data to delineate the surface of an anomalous geologic structure such as a salt dome is disclosed. Seismic wavefields are propagated from near the surface, through the earth, and through the anomalous geologic unit, so that a wavefield encounters the geologic unit at a non-normal incidence angle to generate a converted S- wave. Incidence angles of the direct arrival and converted waves are then determined. The intersection point of each converted S- wave raypath with each direct arrival raypath is determined, to provide a length of the raypaths. Travel times along the raypaths are determined, and a point on the interface of the anomalous geologic unit with the surrounding earth is determined by comparing raypath travel time differences with the recorded arrival time differences.

Claims

1. A method of using mode converted pressure, P- to S- shear wave data to delineate the surface of a geologic unit having an anomalous velocity, comprising the steps of:

(a) fixedly attaching at least two seismic sensors to the outside of a tubing means, said tubing means having a top end and a bottom end, each said sensor having three orthogonal axes of sensitivity;

(b) lowering said bottom end of said tubing means into a wellbore which penetrates said geologic unit and determining the depth of each of said sensors;

(c) calculating said orientations of each of said axes, with respect to an Earth based reference;

(d) causing at least one seismic wavefields to emanate from at least one seismic source points wherein said seismic wavefields are propagated through said geologic unit and the surrounding earth, so that a wavefield encounters said geologic unit at a non-normal incidence angle to generate a converted S- wave, and is observed at said orthogonal axes of sensitivity at said sensors;

(e) separately recording said observed wavefields along each of said orthogonal axes at each of said sensors;

(f) determining said incidence angle φ

_p of said direct arrival wave;

(g) determining said incidence angle φ

_s of said converted S- wave, where

space="preserve" listing-type="equation">φ

.sub.s =sin.sup.-1 (dt/dz)*Vs;

wherein dt is the difference in arrival time of the mode converted shear wave, and wherein dz is equal to the difference in depth between the sensors, and wherein Vs is equal to the shear wave salt velocity;

(h) determining the intersection point of each converted S- wave raypath with the raypath of each direct arrival;

(i) determining the length of said raypaths;

(j) determining the travel times along said converted S- wave raypaths and along said direct arrival raypaths by dividing said length of each raypath by the respective velocity of such raypath; and

(k) determining a point on the interface of said geologic unit with said surrounding earth by comparing the difference between said raypath travel times with the difference between the recorded arrival times of said converted S- wave and said direct arrival wave.

2. The method as defined by claim 1, further comprising the step of determining the interface of said geologic unit with said surrounding earth at a point along said converted S- wave raypath in three dimensions if said direct arrival raypath is in the source-receiver plane.

3. The method as defined by claim 1, wherein said geologic unit is a salt dome.