A method comprising: disposing a toolstring in a cased portion of a borehole penetrating a subsurface formation, wherein the cased borehole portion comprises a metal casing, and wherein the toolstring comprises a module comprising a plurality of acoustic transducers distributed azimuthally around the toolstring and each operable to emit and receive acoustic signals; operating the module to: emit an acoustic signal, from at least one of the transducers, into fluid surrounding the module in the casing; and record data indicative of receipt, by a plurality of the transducers, of acoustic waveforms resulting from interaction of the emitted acoustic signal with the casing, including at least a non-specular diffraction head wave (224; 400) excited by a guided wave (222; 410) that is excited by diffraction of the acoustic signal propagating in the metal casing; and determining acoustic velocity of the guided wave in the metal casing ( V guided) utilizing the recorded data and geometric parameters of the module, characterized in that the recorded data are used to determine: arrival times of the head wave at two or more of the plurality of transducers; and the geometric parameters of the module are used to determine azimuthal offsets of each of the plurality of transducers relative to the at least one transducer from which the emitted acoustic signal was emitted; and a collective outer diameter formed by radially outward extents of each of the plurality of transducers. disposing a toolstring in a cased portion of a borehole penetrating a subsurface formation, wherein the cased borehole portion comprises a metal casing, and wherein the toolstring comprises a module comprising a plurality of acoustic transducers distributed azimuthally around the toolstring and each operable to emit and receive acoustic signals; operating the module to: emit an acoustic signal, from at least one of the transducers, into fluid surrounding the module in the casing; and record data indicative of receipt, by a plurality of the transducers, of acoustic waveforms resulting from interaction of the emitted acoustic signal with the casing, including at least a non-specular diffraction head wave (224; 400) excited by a guided wave (222; 410) that is excited by diffraction of the acoustic signal propagating in the metal casing; and determining acoustic velocity of the guided wave in the metal casing ( V guided) utilizing the recorded data and geometric parameters of the module, characterized in that the recorded data are used to determine: arrival times of the head wave at two or more of the plurality of transducers; and the geometric parameters of the module are used to determine azimuthal offsets of each of the plurality of transducers relative to the at least one transducer from which the emitted acoustic signal was emitted; and a collective outer diameter formed by radially outward extents of each of the plurality of transducers. emit an acoustic signal, from at least one of the transducers, into fluid surrounding the module in the casing; and record data indicative of receipt, by a plurality of the transducers, of acoustic waveforms resulting from interaction of the emitted acoustic signal with the casing, including at least a non-specular diffraction head wave (224; 400) excited by a guided wave (222; 410) that is excited by diffraction of the acoustic signal propagating in the metal casing; and determining acoustic velocity of the guided wave in the metal casing ( V guided) utilizing the recorded data and geometric parameters of the module, characterized in that the recorded data are used to determine: arrival times of the head wave at two or more of the plurality of transducers; and the geometric parameters of the module are used to determine azimuthal offsets of each of the plurality of transducers relative to the at least one transducer from which the emitted acoustic signal was emitted; and a collective outer diameter formed by radially outward extents of each of the plurality of transducers. arrival times of the head wave at two or more of the plurality of transducers; and the geometric parameters of the module are used to determine azimuthal offsets of each of the plurality of transducers relative to the at least one transducer from which the emitted acoustic signal was emitted; and a collective outer diameter formed by radially outward extents of each of the plurality of transducers. azimuthal offsets of each of the plurality of transducers relative to the at least one transducer from which the emitted acoustic signal was emitted; and a collective outer diameter formed by radially outward extents of each of the plurality of transducers.