THE INFLUENCE OF ATTENUATION ON THE STRUCTURE OF HYPERVELOCITY IMPACT CRATERS (Invited Presentation)
However, these observations are selective and are in conflict with many others. Simple craters in coherent media have d = 0.3D and have central shock levels at the top of the parautochthone of only about 5-10 GPa. In complex craters shock levels range from 10 GPa in smaller craters to about 30 GPa in larger. A revised general formula for transient cavity depth in crystalline rocks can be calculated on the basis that: (1) the depth of the transient cavity is determined by attenuation of rarefaction waves that fragment the target until the pressure decays to the dynamic tensile strength of the target (about 200 MPa for crystalline rocks); (2) rim slumping to form peripheral troughs and terraces is progressively more extensive as size increases; (3) the exponent for attenuation in the far field averages -2.5. The proposed formula, by which d = 0.45 D0.75, violates scale proportionality but provides a smooth transition from simple craters to complex craters of intermediate size. It converges towards d = 0.1D at large sizes. An alternative solution would accept variation in the far field attenuation rates. The data suggest an increase in rate from about -2 for small craters towards -3 for the largest for both the initial shock compression and the subsequent rarefaction waves originating at the free surface. Calculations show the direction of such a change is in accord with impact velocity increasing with bolide mass though greater overburden pressure in large craters may be a contributing factor.