Experimental study of P-wave attenuation in partially frozen brine

In order to estimate the amount of methane hydrates (MHs) which form in marine sediments at water depths greater than a few hundred meters, using not only velocity information but also attenuation information can provide much more information about MH-bearing sediments. While the presence of MH increases seismic velocity in the host sediment, recent works on sonic logging data show that sonic waveforms are also significantly affected by the presence of MH. These studies also showed a strong correlation of attenuation with velocity in the MH-bearing sediments. However, the increase of attenuation with increasing velocity is somewhat unintuitive. Thus, it is important elucidate the rock physical mechanism responsible for these phenomena. In this study, we conducted laboratory measurements to explain partially the reason for the physically unrealizable phenomenon. The ice generated from brine was assumed to be methane hydrate, namely, partially frozen brine was considered to be as an analogue for a mixture of methane hydrate and water present in the pore space of hydrate bearing sediments. We observed the variations of a transmitted wave with frequency content of 150-V1000 kHz through a liquid system to a solid-liquid coexistence system, changing its temperature from 20 ,aC to -20 ,aC. As a result, P-wave speed increases with changing in a solid-liquid coexistence system from a liquid system, while P-wave attenuation increases with changing in a solid-liquid coexistence system from a liquid system. Our observations indicate that the interaction in a micro scale of the solid and liquid causes the dissipation of transmitted wave energy.