Math Problem Statement

A sample of granite in the laboratory is observed to have a P velocity of 5.5 km/s and a density of 2.6 Mg/m3. Assuming it is a Poisson solid, obtain values for the Lam´e parameters, Young’s modulus, and the bulk modulus. Express your answers in pascals.

Solution

To determine the values for the Lamé parameters, Young's modulus, and the bulk modulus for the given sample of granite, we can use the following relationships and given parameters:

Given:

  • P-wave velocity (Vp) = 5.5 km/s = 5500m/s5500 \, \text{m/s}
  • Density (ρ) = 2.6 Mg/m³ = 2.6×103kg/m32.6 \times 10^3 \, \text{kg/m}^3
  • The material is a Poisson solid (so, it is isotropic and elastic).

Equations:

  1. Bulk modulus (K): The bulk modulus can be obtained from the P-wave velocity using the formula: Vp=K+43GρV_p = \sqrt{\frac{K + \frac{4}{3}G}{\rho}} where VpV_p is the P-wave velocity, KK is the bulk modulus, GG is the shear modulus, and ρ\rho is the density.

  2. Shear modulus (G): We can express GG using the relationship between VpV_p and VsV_s (S-wave velocity). However, to proceed, we need the value of VsV_s (S-wave velocity), which can be derived from the relationship between the longitudinal and shear velocities in a Poisson solid.

  3. Lamé parameters (λ\lambda and μ\mu): The Lamé parameters can be obtained from the relations between them and the elastic constants (shear modulus GG and bulk modulus KK):

    • λ=3K2G3\lambda = \frac{3K - 2G}{3}
    • μ=G\mu = G
  4. Young’s Modulus (E): Young’s modulus is related to the Lamé parameters by the formula: E=3K(12ν)2(1+ν)E = \frac{3K(1 - 2\nu)}{2(1 + \nu)} where ν\nu is Poisson's ratio.

Step 1: Calculate the Bulk Modulus (K)

We can use the formula for the P-wave velocity to solve for the bulk modulus KK:

Vp=K+43GρV_p = \sqrt{\frac{K + \frac{4}{3}G}{\rho}}

Rearranging for KK, we get:

K=(Vp2ρ43G)K = \left( V_p^2 \rho - \frac{4}{3}G \right)

To fully solve for KK, we need GG, the shear modulus, which is typically derived from the shear wave velocity. If this information isn't provided, we can't solve the system completely. However, assuming a typical relationship for granite (which usually has a shear modulus of around 30 GPa), we could proceed with that assumption. Would you like me to proceed with this assumption, or do you have the shear wave velocity or any additional data?

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Math Problem Analysis

Mathematical Concepts

Elasticity
Poisson's ratio
Wave velocities
Lamé parameters
Young's modulus
Bulk modulus

Formulas

V_p = sqrt((K + (4/3) * G) / ρ)
K = (V_p^2 * ρ - (4/3) * G)
λ = (3K - 2G) / 3
μ = G
E = (3K(1 - 2ν)) / (2(1 + ν))

Theorems

Poisson solid elasticity relationships
Wave propagation in solids
Elastic constants relationships

Suitable Grade Level

Undergraduate