Isopach

source: http://www.fas.org/irp/imint/docs/rst/Sect19/Sect19_6.html

Isopach maps are used in displaying geologic data. Contour lines are used to differentiate strata thickness (strata along the same continuous line have the same thickness). The picture above is a isopach graph of the moon that was created after seismograph studies began during the Apollo 15 flight. These studies determined a 3km layer of ejecta on the moon's surface.