Electrical conductivity models for bubble-bearing marine fine-grained sediments

ABSTRACT

Large sediment-displacing bubbles are widespread in seafloors and typically found in shallow and fine-grained sediments. These bubbles have significant environmental and engineering impacts. Acoustic methods are the most widely used geophysical investigation methods for bubble-bearing sediments, and they have unparalleled technical advantages for low gas content sediments. However, the shielding effect of bubbles on acoustic signals significantly weakens the interpretation ability of acoustic technology for high gas content sediments. Electrical and electromagnetic methods may be able to address this issue, but the setting of gas occupying pore space in traditional electrical conductivity models is not applicable to bubble-bearing sediments. To improve the theoretical basis for the interpretation of bubble-bearing sediments by electrical methods, the bubble Waxman–Smits model (gas content conductivity model) was developed by setting bubbles as nonconductive skeletons within the framework of the Waxman–Smits model (semiempirical model), which is the most widely used conductivity model for cohesive soils. In addition, considering the potential of the Hashin–Shtrikman model (based on the effective medium theory) for joint acoustic–electric inversion, the bubble Hashin–Shtrikman model was developed within the framework of the Hashin–Shtrikman model by setting the bubbles and the sediment matrix as different conductive phases and considering their volumetric and geometric relationships. In the bubble Hashin–Shtrikman model, the bubble content, shape, and orientation are considered. The two bubble-bearing sediment conductivity models are in good agreement with the laboratory experiments (R2 > 98.9%). A comparison of the conductivity model with the geoacoustic model showed that, when the gas content was very low (<1%), the change in the sound velocity caused by bubbles far exceeded the change in conductivity, and with increasing gas content, the rate of change in conductivity caused by the change in gas content gradually exceeded the rate of change in sound velocity.