Geometry

Lajitas, Texas Seismic Station
View of the Chisos Mountains in the background with the array in the foreground. Summer thunderstorms over the Chisos form impressive scenic and seismic displays - and also knock out power for days.

Map of the Lajitas seismic array.
Element locations are shown in blue. A larger scale version is available by selecting the image.

All elements except the center would consist of a single vertical sensor capable of resolving quiet Lajitas background noise over a reasonable band. The center element would consist of a three component element in a standard suface vault.

Massive Cretaceous limestone crops out on the surface (there is no soil), making trenching and excavation operations (impossibly) expensive. All elements were therefore designed to use radio telemetry and be self powered. This also allowed additional elements to be added, or noisy elements to be relocated at little cost.[1]

From Element C2 (TX05) looking toward the array hub
The array hub is on the distant hill top about 2 km away. The bluff exposes the underlying Cretaceous limestone present at each site. Topography such as this makes telemetry over cables impossibly expensive.

This photo was taken during in 1995 when the site was shown to visiting Russians.

The requirement for radio telemetry using VHF frequencies imposed one additional restriction on the geometry. Elements were required to be within VHF transmission range of the concentrator and controller (in this case at the center element). While repeaters were a possibility, they would have added additional complexity to the design.

[1]After analysis of the data, we did move one site that was unusually noisy due to proximity to a road and powerline. In addition, another element was added to handle an additional sensor at the central location.