Gravity effects on water table - USGS

It was on 13th November, 2009 when I joined my friend Brandon Forbes from USGS for the second time in his survey to measure acceleration due to gravity in different places around Tucson.

We did that survey for four different places that day. The water well within the vicinity of El Dorado Hills, Town Homes at Speedway and Caribe was the hub for east Tucson relative to which gravity was measured at other places. Other three places where we measured acceleration due to gravity were Bear Canyon & Snyder, Sabino Canyon, and at Wilmot and Golf Links Road.

Photo-1: @ Bear Canyon and Snyder

Photo-2: @ Sabino Canyon

Photo-3: @ Wilmot and Golf Links

Acceleration due to gravity was first measured at the hub and then at the three different places simultaneously. Deviation in value of "g" compared to that in hub gives the change. Absolute gravity measurement at the hubs were done at certain intervals using the absolute gravity meter.

Photo-4: Absolute Gravity Meter

Second measurement was again done in all the three places almost within one hour. Before each measurement, we needed to calibrate the gravity meter at the hub. Gravity meter was used to measure acceleration due to gravity at each place and it was measured in mgal. (Gal referred as "galileo" is equal to 1 centimeter per second squared source).This measurement is a field work for a project at USGS Arizona Water Science Center which is studying the amount of aquifer storage in Tucson.

Photo-5: Tucson water under concern

Ground water depletion and land subsidence are said to be occurring in Tucson. So, this study is tended to make sure that the average annual aquifer storage change does not become negative denoting that ground water has undergone depletion. This aquifer storage loss can cause damage to infrastructure due to land subsidence. The project going on is based on the fact that gravity is affected by mass and distance - mass change being the change in water table and distance change being the land subsidence. Thus by removing the effect of change in distance, changes in gravity is used to measure changes in aquifer storage.

A related study dealing on land subsidence and aquifer system compaction in south central Tucson was done between 1987 to 2005, report of which can be accessed from here.