A method of investigating a multi-string wellbore

A method using sonic tool data to investigate a multi-string well bore. Sonic measurements are performed along a depth interval in a wellbore to provide sonic data for multiple locations along the wellbore 310. The sonic data is processed to obtain indications as a function of depth of at least one of an energy spectrum, a semblance projection, a slowness dispersion projection, an attenuation dispersion projection, and a wavenumber dispersion projection (figures 13a-13f). These indications are inspected to locate a shift 340 at a particular depth indicating a transition in the material present in at least one annulus (fig. 13a, 410a, 410b) of the multi-string wellbore. The processing can include applying a bandpass filter 320 to the sonic data, and applying semblance processing to the filtered data 334. This can be used to monitor the presence of cement or water in an annulus.