Analysis of legacy gravity data reveals sediment‐filled troughs buried under Flathead Valley, Montana, USA.

Shallow, dominantly silt‐ and clay‐filled erosional troughs in Quaternary sediments under the Flathead Valley (northwestern Montana, USA) are very likely to be hydraulic barriers limiting the horizontal flow of groundwater. Accurately mapping them is important because of increasing demand for groundwater. We used a legacy Bouguer gravity map measured in 1968. The directional derivatives of the map are computed, and the map was enhanced by implementing edge detection tools. We produced generalized derivative, maximum horizontal gradient, total gradient and tilt gradient maps through two‐dimensional Fourier transform analysis. These maps were remarkably successful in locating buried troughs in the northern and northwestern parts of the study area, closely matching locations determined previously from compiled borehole data. Our results also identify hitherto unknown extensions of troughs and indicate that some of the buried troughs may be connected. [ABSTRACT FROM AUTHOR]