Retrievable Fiber Optic VSP System Depth Correlation

The patent introduces a depth correlation unit as an intermediary device that uses formation reference regions (such as casing collars or distinct geological features) to mediate between the fiber optic cable position and the wellbore depth. This intermediary establishes accurate depth correlations by matching reference features detected by both the depth correlation unit and the DAS system, thereby resolving the depth accuracy issue while maintaining the spatial detection capability of DAS.

The patent replaces traditional mechanical depth measurement systems with an optical-based depth correlation system. Instead of relying on mechanical encoders or depth wheels that may drift or accumulate errors, the system uses optical sensing through the fiber optic cable to detect reference features and correlate depths optically, thereby reducing noise artifacts and improving measurement precision.

The patent implements a feedback mechanism where the depth correlation unit continuously monitors the position of reference features and compares them with the expected positions based on wellbore geometry and fiber deployment. This feedback loop allows the system to detect and compensate for depth errors caused by temperature-dependent index of refraction changes and fiber stretch, thereby maintaining high measurement precision without excessive system complexity.

The patent accounts for parameter changes in the fiber optic cable due to temperature and stress by measuring these parameters directly (temperature via the fiber itself, strain via Brillouin scattering) and using them to correct the depth calculations. This approach maintains measurement accuracy by dynamically adjusting for environmental conditions rather than assuming constant fiber properties.

The patent designs the depth correlation unit to perform multiple functions: it detects reference features (such as casing collars or geological markers), correlates them with the reference log, and provides depth information to the DAS system. By making this single unit multi-functional, the patent achieves high depth correlation accuracy without proportionally increasing system complexity, as the same hardware infrastructure serves multiple purposes.

The patent merges the depth correlation functionality with the existing DAS interrogator and fiber optic cable infrastructure. Instead of adding completely separate depth measurement equipment, the system combines depth correlation capabilities with the acoustic sensing system by using the same fiber optic cable for both acoustic signal transmission and depth reference detection, thereby reducing overall system complexity while improving depth correlation accuracy.