Acoustic imaging using high-frequency ultrasound can be an effective survey method to clarify the distribution of asari clams, which live in a relatively shallow layer of sediment. However, the propagation of ultrasound dramatically changes depending on grain size, resulting in visibility deterioration of acoustic images. In this study, the effects of grain size on the visibility of acoustic images were investigated by experiments using glass beads. Then, the effects were discussed using theoretical attenuation models (Biot-Stoll model and multiple scattering model developed by Schwartz and Plona [J. Appl. Phys. 55, 3971 (1984)]) and time-frequency analysis based on the continuous wavelet transform (CWT). In addition, wavelet shrinkage was applied to improve the visibility of acoustic images. The results suggest that multiple scattering and velocity dispersion strongly affected the sharpness and contrast of acoustic images depending on grain size. In addition, it was found that wavelet shrinkage was effective in reducing speckle noise and increasing the visibility of acoustic images of buried asari clams. (C) 2018 The Japan Society of Applied Physics