Downhole Linear Receiver Array Travel Time Estimation
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Solution Overview
Problem
Existing methods for estimating travel time and move-out velocity using downhole linear receiver arrays face challenges, particularly in accurately determining formation compressional velocity when interbedding occurs, leading to averaging or multi-valued velocity issues, and are hindered by high noise levels.
Innovation Solution
The implementation of the Instantaneous Phase (IP) and Cross Correlation (CC) methods for travel time and velocity estimation, which involve wavefield separation, local slowness computation, and edge detection to track the first arriving energy across the receiver array with high resolution, providing accurate travel time and velocity logs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional travel time picking methods are used, then the process is simple, but the measurement precision deteriorates due to averaging effects in interbedded formations
Solution Approach 1:
The patent segments the travel time estimation process into two distinct phases: coarse travel time picking using traditional methods, followed by fine travel time picking using the Instantaneous Phase and Cross Correlation methods. This segmentation allows the system to achieve high precision in velocity estimation without requiring complete redesign of the entire processing workflow, thereby resolving the contradiction between measurement precision and device complexity.
Solution Approach 2:
The patent performs preliminary coarse travel time picking using traditional methods before applying the more sophisticated Instantaneous Phase and Cross Correlation methods. This preliminary action provides initial velocity estimates that guide subsequent high-precision processing, reducing the computational burden and complexity while maintaining high measurement precision in the final velocity logs.
2Measurement precision
If traditional semblance methods are used, then the computational process is straightforward, but the measurement precision deteriorates due to noise sensitivity
Solution Approach 1:
The patent introduces the Instantaneous Phase method as an intermediary approach between traditional picking and Cross Correlation. This intermediary method uses phase information from the wavefield to estimate travel times, which is more robust to noise than amplitude-based methods. The phase information serves as a mediator that preserves signal characteristics even when noise contaminates the amplitude data, thereby improving velocity estimation accuracy in noisy conditions.
Solution Approach 2:
The patent transitions from amplitude-based travel time picking to phase-based and correlation-based methods. By changing the parameter used for travel time estimation from amplitude to phase and correlation coefficients, the system becomes less sensitive to noise while maintaining computational feasibility. This parameter change is fundamental to resolving the contradiction between measurement precision and noise sensitivity.
3Measurement precision
If receiver array aperture is increased to improve velocity resolution, then the vertical resolution improves, but the device complexity increases
Solution Approach 1:
The patent implements dynamic receiver spacing within the linear array, where receivers are positioned at varying intervals rather than uniform spacing. This dynamic configuration allows the array to achieve high vertical resolution for velocity estimation without requiring a uniformly large aperture. The dynamic spacing optimizes the sampling of wavefield information across different depths, resolving the contradiction between resolution and aperture size.
Data Source
AI summary
A method for borehole measurements may comprise receiving one or more signals from a linear receiver array, computing an arctan of a Hilbert Transform, isolating a first arriving energy, selecting a reference instantaneous phase on a reference receiver, finding the reference instantaneous phase for the linear receiver array, computing a relative travel time shift, combining a reference pick time with a relative time, and determining a travel time. A system for borehole measurements comprise a conveyance, a bottom hole assembly attached to the conveyance, a linear receiver array, wherein the linear receiver array is disposed on the bottom hole assembly, and a computer system connected to the linear receiver array.