# 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)[1] gave some equations from Hughes 1998[2] and Davis 1986[3]. 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[4]). 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.

## References

1. Stewart, S (1999), Seismic interpretation of circular geological structures, Petroleum Geoscience 5, 273–285.
2. Hughes, D, 1998, The mass distribution of the crater-producing bodies. In Meteorites: Flux with time and impact effects, Geological Society of London Special Publication 140, 31–42.
3. Davis, J, 1986, Statistics and data analysis in geology, John Wiley & Sons, New York.
4. Chapman & Morrison 1994, Impacts on the Earth by asteroids and comets: assessing the hazard. Nature 367, 33–40.