Impact crater

Impact craters are a rare type of circular feature in seismic data. If you find a round thing in your seismic, it might help to know what the probability of it being an impact structure is.

The Steen River crater in northern Alberta is a Cretaceous-age impact buried in the subsurface; there are hydrocarbons associated with the feature.

Compute probability of a crater
 Stewart (1999) gave some equations from Hughes 1998 and Davis 1986. The probability P of encountering r craters of diameter 1 &le;d &le; 500 or more in an area A over a time period t years is given by


 * $$P(r) = \mathrm{e}^{-\lambda A}\frac{(\lambda A)^r}{r!}$$

where


 * $$\lambda = t n \ $$

and


 * $$\log n = - (11.67 \pm 0.21) - (2.01 \pm 0.13) \log d $$

These equations are built into the Wolfram Alpha Widget at right (requires JavaScript). You will probably want to leave the number of craters at 1; enter the size of structure you are concerned with — perhaps you have observed a circular feature in a seismic reflection survey; give the area of interest, and the time period. Take note of the caveats, given below.

Example
The probability of an impact structure 1 km or greater in diameter in an Albertan township (36 square miles = 93 km2) in the Cenozoic (65 Ma) is 0.012.

Caveats
The default diameter is rather small: the minimum size required to reach the ground intact is estimated to be 100–200 m diameter, resulting in a crater 2–3 km in diameter (Chapman & Morrison 1994 ). Bolides can break up, however, resulting in smaller craters.

Clearly, you need to think a bit about depositional environments and periods of uplift and erosion. Deep marine environments don't record small structures. Mountains won't result in a nice crater except for large bolides. When you consider erosion, it helps to know that, while the initial (transient) crater may have a depth/diameter ratio of 0.3, a typical final ratio is 0.1.

Also, note that estimates of terrestrial impact flux &lambda; vary by at least a factor of two.