# 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

where

and

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 km^{2}) 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

- ↑ Stewart, S (1999), Seismic interpretation of circular geological structures, Petroleum Geoscience 5, 273–285.
- ↑ 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. - ↑ Davis, J, 1986,
*Statistics and data analysis in geology*, John Wiley & Sons, New York. - ↑ Chapman & Morrison 1994, Impacts on the Earth by asteroids and comets: assessing the hazard. Nature 367, 33–40.