MODELING COMPRESSIONAL WAVE VELOCITY AND ATTENUATION IN CARBONATE SEDIMENTS

The Biot poroelastic theory is used to model ultrasonic (1 MHz) compressional wave velocity (V(p)) and specific attenuation (Q(p)-1) for carbonate sediment sequences from three Deep Sea Drilling Project (DSDP) sites 288, 289, and 316 on the Ontong-Java Plateau in the western Pacific ocean. The sediments are assumed to be represented by a granular frame containing tortuous capillary tubes of a constant radius. Experimental data for the variation of shear velocity V(s) and bulk density rho with depth are used as input parameters for the model. Assumptions of the frame Poisson's ratio sigma-b, grain bulk modulus K(g), and tortuosity alpha are necessary. The low-frequency asymptote of the Biot theory is adequate in modeling the variation of V(p) with depth for sites 288 and 289. Modeled velocities agree to within 8% of experimental. The sensitivity of the modeled velocities to the uncertainty in the input parameters is investigated. The equations of motion are partially differentiated with respect to each parameter and the resulting equations are used in conjunction with each parameter's uncertainty or error. The uncertainty in the modeled velocities is on average 11% and is mostly dependent upon the uncertainties in K(g) and sigma-b. The assumption of constant sigma-b for the chalk-limestone sequences from sites 288 and 289 is then tested. It is found to remain constant at a value of 0.2 for the chalk, and decreases to approximately 0.1 for the transition to limestone. Modeled values for Q(p)-1 are found to be sensitive to sigma-b and rho, in addition to the pore space parameters, and are considerably smaller than the experimental observations from sites 288, 289, and 316.