<p>A highly prevalent scenario in geosteering operations is a wellbore positioned adjacent to a formation interface without penetrating it. To date, no systematic analysis has been conducted on the acoustic logging responses of this typical heterogeneous formation setting. To fill this research gap, the responses of azimuthal acoustic logging-while-drilling (LWD) in fluid-filled wellbores that do not penetrate the adjacent formation interfaces are numerically simulated, using a 3D Cartesian coordinate finite-difference algorithm. An improved slowness-time-coherence (STC) method is adopted to process array waveforms acquired at different azimuths, and the waveform characteristics as well as circumferential formation velocity inversion patterns are systematically analyzed for four typical formation assemblages (shale-sandstone, sandstone-shale, sandstone-carbonate, and carbonate-sandstone) with varying wellbore-to-interface distances. The results demonstrate that the azimuthal acoustic LWD responses are strongly dependent on both the wellbore-to-interface distance and formation velocity. When the wellbore is in close proximity to the interface, the acquired waveforms are predominantly affected by the properties of the high-velocity formation. As the wellbore-to-interface distance increases, the logging responses are dominated by the properties of the formation immediately surrounding the wellbore, and gradually converge to the response characteristics of a homogeneous formation. The derived circumferential formation velocity distribution maps can be effectively used to identify the presence and azimuth of high-velocity formations adjacent to a wellbore embedded in a low-velocity formation, but exhibit limited capability for the detection of low-velocity formations near a wellbore surrounded by a high-velocity formation. A solid theoretical foundation is provided by this work for the field application of azimuthal acoustic LWD technology in geosteering operations.</p>