Applications of laboratory velocity and attenuation data to studies of the Earth's crust
Abstract
The interpretation of seismic data depends on laboratory studies of velocities and attenuations. Constraints on the Earth's composition are provided by comparing laboratory and field velocities, and the use of attenuation data in the same manner is promising. The empirical equation relating velocity (V) to confining pressure (P)$$\rm V(P) = A(P/100\ MPa)\sp{a} + B(1 - e\sp{-bP})$$is proposed, where A, a, B, and b are adjustable parameters which may be related to crack structure. This equation is used to curve fit V$\sb{\rm P}$ and V$\sb{\rm S}$ data for a range of samples and gives well-constrained parameters. Expression of a complete V-P data set is thus reduced to four constants, since data interpolation and extrapolation are accurately performed. Pressure derivatives, velocity anisotropies and elastic constants as functions of pressure are simply determined from the equation. Oceanic crustal attenuation (or Q) is investigated by measuring compressional wave attenuations as a function of confining pressure in several basalt and ophiolite samples. An ultrasonic pulse-echo spectral ratio technique has been developed to measure attenuations to pressures of 500 MPa. Applying pressure closes cracks, which reduces the attenuation, and the mineralogy determines the attenuation above 200 MPa. Because the oceanic crust is at pressures below about 200 MPa, Q should increase with depth. For oceanic basalts, a high porosity and secondary mineral content causes high attenuations. Q variations in Layer 2 may thus reflect compositional changes. An inverse relationship between velocity and attenuation is observed, meaning that the same factors which cause a low V$\sb{\rm P}$ also give a low Q$\sb{\rm P}$. V$\sb{\rm P}$ may thus be used to estimate basalt unit Q's. Saturation of some basalts increases the attenuation at pressures below 200 MPa and increases the sensitivity of attenuation to pressure, an important consideration for Layer 2 pressures. Based on ophiolite attenuation data, a Q profile for the ocean crust is constructed. It shows a slow rise in Q with depth through the pillow basalts of Layers 2A and B, a Q increase with depth due to progressive metamorphism in the sheeted dikes of Layer 2C, a dramatic drop in Q in the gabbros of Layer 3, and an increase in Q at the Moho. The Q contrast at the Moho could be altered by Layer 3 serpentine diapirs or by serpentinization of the upper mantle, since serpentinization greatly affects Q.
Degree
Ph.D.
Advisors
Christensen, Purdue University.
Subject Area
Geophysics
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