# Impact crater

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

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 λ vary by at least a factor of two.