1. A scanning method which is a method of identifying a change in the density of a pipeline located underwater, said change representing a difference between the density of said pipeline at a first location and the density of said pipeline at a second location adjacent said first location, the method comprising the steps of: a. arranging a source of ionizing radiation and an array of N radiation detectors Dn, where n is an integer from 1 to N, capable of detecting said radiation in such a way that i. radiation is emitted from the source in the direction of the array of radiation detectors, ii. radiation is emitted from the source towards any one of said radiation detectors along a radiation path, said radiation path being defined at each end by the source and the area of each detector facing said source; iii. up to N of said radiation paths may pass through said pipeline; and iv. said source and said detector array are located in fixed positions relative to one another and movable along an arcuate scanning path around said pipeline; b. positioning the source and detector array at a known, position p0 on said scanning path; c. acquiring count data Cnx from each detector Dn, for n=1 to N, for a predetermined period of time, said count data being related to the number of photons of radiation emitted by the source which have been detected by said detector during the predetermined period of time; d. repeating steps b and c at a plurality of different positions px on the scanning path, where x is an integer from 1 to X; e. optionally adjusting each Cnx according to a calibration; f. optionally converting each Cnx by means of a fan to parallel beam conversion algorithm to a converted Cnx; g. for each detector Dn, calculating normalised Cnx values for each Cnx for n=1 to N and for x=1 to X, h. collating each normalised Cnx value in a matrix with rows and columns corresponding to n and x; i. analyzing the matrix to detect a pattern within the matrix of normalised Cnx values which are higher or lower than the mean counts value; and inferring from said pattern the presence of a change in the density of said pipeline at a location lying on at least one of said radiation paths.