Field Evidence of Frequency-dependent Apparent Body-wave Velocity Transition

Frequency-dependent fluctuations of body-wave traveltimes in heterogeneous media have been extensively documented at global and laboratory scales, yet they have never been systematically observed in geophysical prospecting. The difficulty faced by prospecting stems from a fundamental acquisition challenge: the inherent trade-off between high-frequency signals and investigation distance. Here we present the first field-scale observation of frequency-dependent apparent body-wave velocity transition using a novel high-power acoustic source combined with encoding-decoding and multi-stacking strategy. This system enables recording of unprecedented high-quality crosshole waveforms with dominant frequencies up to 1.1 kHz over a 105 m span. The results reveal a systematic decrease in apparent velocity with increasing frequency, which is physically attributed to a scattering regime transition from Rayleigh to Mie scattering. The amplitude of velocity variation is jointly controlled by heterogeneity within the first Fresnel zone and fast-path effect. To fully exploit this newly resolved physical mechanism, we develop a frequency-hierarchy quasi-wave equation traveltime inversion (HQWTI) method that explicitly incorporates multi-frequency traveltimes into tomographic imaging. Application to the field data produces a meter-scale velocity tomogram, achieving an order-of-magnitude resolution improvement over conventional ray-theory traveltime tomography that assumes infinite-high frequency. The enhanced resolution further corroborates the physical validity of the observed velocity transition. With increasing signal frequency, imaging resolution improves by approximately twofold in both horizontal and vertical directions. Considering material-property-based intrinsic attenuation and ambient noise, the effective upper usable frequency limit is 1.4 kHz for a 100 m propagation distance in sedimentary rocks.