Girth weld inspections play a key part in the maintenance and management of numerous different types of pipe that are mainly used within the oil & gas industry. The most modern technique for girth weld inspection exploits full matrix capture (FMC) with the total focusing method (TFM), utilising two probes in a combined pulse-echo and pitch-catch set-up. This set-up has the advantage of allowing multiple modes to be carried out within a single inspection, with the pitch-catch part requiring a precise measurement of the probe separation distance (PSD) to be determined for accurate TFM image reconstruction. This distance is the lateral measurement between the first elements of each probe. The currently accepted method for calculating this distance is through visual means (with the use of a physical measurement ruler), which suffer from operator error, the ruler not being calibrated and difficulties determining the position of the first element. This paper presents an ultrasonic measurement technique for accurately calculating the PSD by exploiting the lateral wave or the shear wave backwall response within the pitch-catch mode A-scan analysis.

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Ultrasonic inspection of through-transmission is limited due to the inability to obtain defect depth information. Loss of signal is used as the only indicator, providing lateral defect information. This is often a problem in ultrasonic inspection. Radiographic acquisition techniques, where the X-ray source acts as the transmitter and the detector as the receiver, are conceptionally similar to ultrasonic through-transmission. In the latter, the tomography back-projection method is used to reconstruct images of an object that has been subjected to a minimum of 180° of rotation, to allow for full coverage of the item. In this paper, a novel approach based on back-projection is presented to improve image resolution and defect detectability. Two ultrasonic transducers in through-transmission configuration are utilised to capture data for image processing. The rotation of the transmitter and receiver is not possible in this set-up and, therefore, the reconstruction relies on the artificial generation of a limited rotation. Two probes are aligned either side of the material and are used to gather the ultrasonic signals. These signals are processed before the reconstruction algorithm is applied to them. Various processing and imaging reconstruction algorithms are explored, building on the basic back-projection method to obtain images that are better focused. This technique could be used within materials where there are high attenuation levels and, therefore, traditional pulse-echo is not feasible.

Ultrasonic through-transmission data processed using the back-projection algorithm offers depth and lateral information about a defect beyond the capabilities of current through-transmission techniques. This technique was trialled on a carbon steel block containing side-drilled holes. Imaging artefacts can arise from the use of the backprojection algorithm, due to applying a weighting of one to each pixel, irrespective of how much of the pixel is intersected by the beam. Noise can also occur within the image where there are few intersections of the pixels made. This is seen at the edges of the image. In this paper, a novel back-projection technique utilises the weighting of pixels, dependent on the normalised weight of the beam that intersects them, to reduce any artefacts that occurred previously due to the backprojection algorithm. This paper also explores the use of the algebraic reconstruction technique (ART) algorithm for noise removal, thus increasing the sharpness of the defect.