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Method to Calibrate Orientation and Position of a Downhole Passive Seismic Array

IP.com Disclosure Number: IPCOM000245385D
Publication Date: 2016-Mar-04
Document File: 4 page(s) / 291K

Publishing Venue

The IP.com Prior Art Database

Related People

Jeffrey Bailey: AUTHOR

Abstract

The known locations of surface or downhole acoustic events may be used to calibrate the orientation and/or position of the receivers of a Passive Seismic Array. The method is to calculate multiple correlation matrices of time-windowed datasets from a triaxial receiver and then to rotate or translate these matrices to determine the maximum alignment between the principal eigenvalues and the direction vectors to each of the acoustic source locations. The orientation or position with the highest alignment is the most likely value. This method is a robust means to determine the receiver orientations, and it can be used to estimate positions if so required.

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Method to Calibrate Orientation and Position of a Downhole Passive Seismic Array

Abstract

The known locations of surface or downhole acoustic events may be used to calibrate the orientation and/or position of the receivers of a Passive Seismic Array. The method is to calculate multiple correlation matrices of time- windowed datasets from a triaxial receiver and then to rotate or translate these matrices to determine the maximum alignment between the principal eigenvalues and the direction vectors to each of the acoustic source locations. The orientation or position with the highest alignment is the most likely value. This method is a robust means to determine the receiver orientations, and it can be used to estimate positions if so required.

Description

A Passive Seismic Array comprises two or more triaxial acoustic sensors (geophones or accelerometers) located in a wellbore placed in a subsurface location. More generally, such arrays have also been disposed on the surface of the earth, or perhaps buried at a shallow depth. Such arrays are often used to locate acoustic sources generated in the subsurface formation, of either active or passive source type. The source locations are determined relative to the position and orientation of the sensors, so therefore it is important to know these parameters for the array elements.

A method has been devised to calculate the orientation and, if necessary, the downhole locations of the array receivers, considering all of the data measurements at the same time. This method is based on an eigenvalue analysis of the waveforms for short calibration time windows, using the known locations of the noise sources for the calibration. This method has been used with good results.

In the results presented below, each of five receivers were relocated using nine acoustic events, including six string shots in one well and induced events in three other wells. These locations are identified with black dots in the charts below. The zero location of the wellbore, or assumed location of the receivers, is shown with a large blue diamond. The white crosses show the preliminary calculated relocations. The method used to calculate these relocations includes the following steps:

1. Perform a hodogram analysis of the event in the horizontal plane.

2. Determine the eigenvalues and eigenvectors of the E-N cross-correlation matrix.

3. Choose the eigenvector corresponding to the largest eigenvalue. This vector has unit length.

4. Form a grid of Easting and Northing coordinates. Grid spacing is arbitrary, but 2 meters was used in this analysis.

5. For each grid point, calculate the direction vector from the grid location to the known location of the event. Normalize this direction vector.

6. Compute the dot product (or projection) of the eigenvector on this direction vector.

7. Weight this value by the event linearity parameter, normalized such that the weights sum to 1.0.

8. Sum the weighted dot product results.

9. Comp...