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HIGHER ORDER TORSIONAL MODES

For fully saturated porous cylinders, the factor that determines the torsional modes of propagation is m11(r) in (46). The critical factor here is the Bessel function J2(js) and, specifically, the whereabouts of its zeroes. One source of this information, to five figure accuracy, is the reference of Abramowitz and Stegun (1965), which provides not only the location of the zeroes j2,n, but also the values of the corresponding derivatives J2'(j2,n). Having these derivatives is useful for improving the accuracy of the zeroes with a Newton-Raphson iterative method, based on j2,n = j2,nold - J2(j2,nold)/J2'(j2,nold). This approach gives a very rapid improvement to the values of the j2,n's within 2 to 3 iterations. The results to order n=3 are shown in TABLE 1.

1.5

TABLE 1.The zeroes j2,n of J2(z) as a function of the order n of appearance along the real axis.

1.0

Having already understood the zeroth order contributions to the dispersion relation (detT) due to zeroes of ksr and ksr*, we are now free to consider that neither of these factors vanishes for the higher order modes. This assumption permits us to factor these wavenumbers in or out of the determinant whenever it is convenient to do so. In particular, we note that the first two columns of (detT) would have a common factor of (which could then be safely eliminated) if we first multiply the bottom row by a factor of . Having made these simplifications, we find after also eliminating a common factor of -1 from the top row, and -ksr from the third column.

Expanding the determinant along the third column, we have Some elementary consequences of this equation are: (a) As so there is no liquid left in the system, J1(ksrR1) and like R1, while Y1(ksr*R1) and like 1/R1. So the dispersion relation is always satisfied in the limit when J2(ksr*R2) = 0, which is exactly the condition for the fully dry cylinder as expected. (b) If , then the first determinant vanishes identically. The second determinant does not vanish in general since it approaches the Wronskian , so the condition becomes ksrJ2(ksrR1) = 0, again as expected. (c) The special case of does not affect these conclusions, as both J1(ksrR1) and in this limit, as they should. (d) The only case that is missing from (detT3) is the one for . But this multiple zero of the original dispersion relation (detT) was eliminated when we removed two factors of (ksr*)2 from the first and second column in the first step of our simplification of the dispersion relation - a step which is always legitimate except when .

We conclude that, with the one trivial exception just noted, these simplifications have kept the basic nature of the dispersion relation intact.

Next: Lower frequency results Up: Berryman and Pride: Cylinder Previous: ELEMENTARY TORSIONAL MODES
Stanford Exploration Project
11/11/2002