Borehole Gravimeter Using Optical Interferometry
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Solution Overview
Problem
Conventional gravimeters face challenges in accurately measuring gravitational acceleration in rugged borehole environments due to high temperatures, dynamic conditions, and the weak nature of gravitational force, leading to insufficient sensitivity and reliability.
Innovation Solution
A gravimeter system with a proof mass constrained by springs and an optical interferometer that measures displacement using a light path change, operating in an open loop mode to enhance sensitivity and reliability, and a processor to determine spatial displacement and gravitational acceleration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional gravimeters are used in borehole environments, then gravitational acceleration can be measured, but measurement precision deteriorates due to high temperatures, dynamic conditions, and shock vibrations
Solution Approach 1:
The patent replaces conventional mechanical measurement systems with an optical interferometry system. The proof mass displacement is measured using optical interference patterns rather than mechanical contacts, eliminating the sensitivity to shock and vibration that plagues mechanical gravimeters in borehole environments. The optical system measures displacement through light path changes without mechanical coupling to the moving mass.
Solution Approach 2:
The patent introduces an optical interferometer as an intermediary measurement system. Instead of directly measuring gravitational force with sensitive mechanical components, the system uses optical fields as intermediaries to detect the position of the proof mass. The interferometer converts mechanical displacement into optical phase changes, providing indirect but more robust measurement in harsh environments.
2Measurement precision
If conventional gravimeters operate in high temperature borehole environments, then gravitational measurements can be obtained, but sensitivity decreases due to thermal noise and dynamic conditions
Solution Approach 1:
The patent replaces mechanical displacement sensing with optical interferometry, which is less sensitive to thermal expansion and mechanical drift. The optical system measures relative displacement through interference fringes, maintaining sensitivity even when absolute dimensions change due to temperature variations in the borehole environment.
3Measurement precision
If open loop operation mode is used, then measurement sensitivity improves, but device complexity increases due to additional optical components
Solution Approach 1:
The patent divides the optical interferometer into distinct functional modules: light source, beam splitter, reference mirror, measurement mirror attached to proof mass, and detector. This segmentation allows each component to be optimized independently and facilitates assembly and alignment in the borehole tool configuration.
Solution Approach 2:
The optical interferometer serves multiple functions: it measures proof mass displacement, provides reference for gravitational acceleration calculation, and operates in the open loop mode to maximize sensitivity. The same optical system is used for both positioning detection and gravitational field measurement, reducing the need for separate sensing systems.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system achieves precise measurement of gravitational acceleration with a noise floor of 1 microGal, improving sensitivity and repeatability, and allows for multi-directional measurements, facilitating accurate characterization of underground formations.
Implementation Method 1
The optical interferometer is configured to generate a light path from a light source to a reflective surface on the proof mass. A processor determines spatial displacement of the proof mass from a reference position to a position of gravitational equilibrium by measuring a change in length of the light path.
Implementation Method 2
The gravimeter includes a proof mass that is constrained by a spring and that is displaceable in response to gravitational acceleration.
Implementation Method 3
The proof mass is displaceable in response to gravitational acceleration. The processor determines spatial displacement of the proof mass from a reference position to a position of gravitational equilibrium.
Data Source
AI summary
A gravimeter, a gravimeter system, and a method for measuring gravitational acceleration within a borehole are described herein. The gravimeter includes a proof mass that is constrained by springs and an optical interferometer for measuring displacement of the proof mass. The optical interferometer generates a light path from a light source to a reflective surface on the proof mass. Spatial displacement of the proof mass from a reference position to a position of gravitational equilibrium is determined by measuring a change in length of the light path. In turn, gravitational acceleration can be determined from the spatial displacement of the proof mass. A number of such gravimeters can be used in a gravimeter system to make measurements of gravitational acceleration in variety of different directions.