A method for scanning a structure (6, 106, 207) to detect differences in density between different parts of the structure (6, 106, 207), the method comprising: providing an apparatus (1, 101, 201) comprising a source of gamma radiation (2, 102, 204) and a plurality of detectors (3a - 3g, 103a - 103h) arranged to detect gamma radiation emitted by the source (2, 102, 204); positioning the structure (6, 106, 207) between the source (2, 102, 204) and the detectors (3a - 3g, 103a - 103h); and rotating the detectors (3a - 3g, 103a - 103h) and the source (2, 102, 204) in a fixed relationship to each other about an axis of rotation located between the detectors (3a - 3g, 103a - 103h) and the source (2, 102, 204) to scan the structure (6, 106, 207) and generate scan data; wherein the plurality of detectors (3a - 3g, 103a - 103h) are arranged in a linear array, the linear array being substantially parallel to the axis of rotation, characterised in that the structure (6, 106, 207) is underwater in the sea and the method comprises deploying the apparatus (1, 101, 201) underwater in the sea to scan the structure (6, 106, 207), and wherein the method further comprises one or both of the following processes to identify a defect: i) rotating the detectors (3a - 3g, 103a - 103h) and the source (2, 102, 204) through an angle of 360 degrees or more, comparing data from two scan positions separated by 180 degrees, and determining, from the comparison, where a defect lies in relation to the axis of rotation, with the defect appearing more blurred when it is located on the same side of the axis of rotation as the source than when it is located on the opposite side of the axis of rotation to the source; ii) providing a second source of gamma radiation, providing a second plurality of detectors arranged to detect gamma radiation emitted by the second source, positioning a part of the structure identified as having a potential defect in an analysis of the scan data between the second source and the second detectors; and rotating the second detectors and the second source in a fixed relationship to each other about an axis of rotation located between the second detectors and the second source; wherein the second plurality of detectors are arranged in an arcuate array, the arcuate array being in a plane perpendicular to the axis of rotation. providing an apparatus (1, 101, 201) comprising a source of gamma radiation (2, 102, 204) and a plurality of detectors (3a - 3g, 103a - 103h) arranged to detect gamma radiation emitted by the source (2, 102, 204); positioning the structure (6, 106, 207) between the source (2, 102, 204) and the detectors (3a - 3g, 103a - 103h); and rotating the detectors (3a - 3g, 103a - 103h) and the source (2, 102, 204) in a fixed relationship to each other about an axis of rotation located between the detectors (3a - 3g, 103a - 103h) and the source (2, 102, 204) to scan the structure (6, 106, 207) and generate scan data; wherein the plurality of detectors (3a - 3g, 103a - 103h) are arranged in a linear array, the linear array being substantially parallel to the axis of rotation, characterised in that the structure (6, 106, 207) is underwater in the sea and the method comprises deploying the apparatus (1, 101, 201) underwater in the sea to scan the structure (6, 106, 207), and wherein the method further comprises one or both of the following processes to identify a defect: i) rotating the detectors (3a - 3g, 103a - 103h) and the source (2, 102, 204) through an angle of 360 degrees or more, comparing data from two scan positions separated by 180 degrees, and determining, from the comparison, where a defect lies in relation to the axis of rotation, with the defect appearing more blurred when it is located on the same side of the axis of rotation as the source than when it is located on the opposite side of the axis of rotation to the source; ii) providing a second source of gamma radiation, providing a second plurality of detectors arranged to detect gamma radiation emitted by the second source, positioning a part of the structure identified as having a potential defect in an analysis of the scan data between the second source and the second detectors; and rotating the second detectors and the second source in a fixed relationship to each other about an axis of rotation located between the second detectors and the second source; wherein the second plurality of detectors are arranged in an arcuate array, the arcuate array being in a plane perpendicular to the axis of rotation. i) rotating the detectors (3a - 3g, 103a - 103h) and the source (2, 102, 204) through an angle of 360 degrees or more, comparing data from two scan positions separated by 180 degrees, and determining, from the comparison, where a defect lies in relation to the axis of rotation, with the defect appearing more blurred when it is located on the same side of the axis of rotation as the source than when it is located on the opposite side of the axis of rotation to the source; ii) providing a second source of gamma radiation, providing a second plurality of detectors arranged to detect gamma radiation emitted by the second source, positioning a part of the structure identified as having a potential defect in an analysis of the scan data between the second source and the second detectors; and rotating the second detectors and the second source in a fixed relationship to each other about an axis of rotation located between the second detectors and the second source; wherein the second plurality of detectors are arranged in an arcuate array, the arcuate array being in a plane perpendicular to the axis of rotation.