Research on fish-top measurement methods for fallen fish based on ultrasonic echo imaging

• Reduce data processing volume for traditional full-focusing methods. • Evaluate measurements of planar sound pressure data and contour eccentricity. • Characterize the relative position between fish head contour and wellbore. • Enhance fish head contour integrity and measurement accuracy. Accurate measurement of the fallen fish within the casing is crucial for fishing operations. However, existing methods face challenges in analyzing and measuring under the condition of eccentric fish top. This study proposes and validates an ultrasonic C-scan measurement method for determining the position and contour-related location parameters of the fallen fish in cased wells. Taking a fractured 127 mm drill pipe inside a 244.5 mm casing as a case study, the correlation between key imaging parameters—such as sound source frequency (20–50 kHz) and detection distance (0.15–0.85 m)—and echo amplitude is systematically investigated. Under the given model conditions, optimal imaging performance is achieved. Measurement results show that when the screening threshold is set at 75%, the contour integrity of the fish top reaches 82.4%. For offset fallen fish, the deviation in the offset vector measured by the centroid method is less than 20%, effectively characterizing the position of the fish top. This work not only implements an ultrasonic C-scan measurement method with fused threshold screening based on array acoustic pressure measurements, achieving quantitative output of the contour integrity and eccentricity state of the fallen fish, but also establishes a measurement model from acoustic pressure data to location parameters and a performance evaluation framework based on ultrasonic numerical simulation results for the casing–fallen fish system. This lays a theoretical and practical foundation for efficient fishing operations in complex wellbore environments, while also providing reliable technical support for the development of intelligently guided fishing tools.