BZ-FA: A Cross-Modality Registration Method for 3-D Borehole Representation in Well Logging.

The time-of-flight (ToF) and amplitude of an ultrasonic echo signal are critical parameters employed to build a 3-D model of an underground borehole in an imaging logging system. However, due to different orientations of the initial measurement or repeated measurements, the point cloud generated by the ToFs of echo signals may not register the corresponding intensities produced by the signal amplitudes. This study proposes a borehole zone feature alignment (BZ-FA) framework to perform cross-modality registration between the point clouds of the borehole walls and their corresponding intensities. First, point clouds and intensities produced by the ultrasonic signal are mapped onto a plane, which is transformed into ToF and amplitude images to align the features of different modalities. The 2-D-to-3-D registration is converted into an image matching problem. Then, we build a mathematical optimization model where the optimization target is to minimize the difference between ToF images and raw amplitude images. Finally, a multi-orientation search (MOS) method is proposed, which performs a search for the optimal shift matrices to achieve registration effectively and accurately. We manufactured an ultrasonic imaging logging (UIL) tool and collected point cloud data for an entire production well to verify our proposed method. The experimental results demonstrated that the proposed method can carry out this cross-modality registration task effectively and yield intuitive models to help comprehend the construction of borehole walls in the well logging.