Deep within the earth beneath our feet lies a silent story. This story affects the very ground we walk on. It’s a story the Southwest Extensometer helps us to understand.
The Southwest Extensometer is more than just a piece of equipment. It is a window into the hidden world of land subsidence. Land subsidence is the sinking of the Earth’s surface. This phenomenon occurs due to underground movements.
Here in the Houston-Galveston area, the primary cause of land subsidence is groundwater withdrawal. Imagine huge amounts of water being pumped out of the aquifer. The clay layers compact and settle. This causes the ground above to sink. Over time this creates a cone-shaped depression.
The Harris-Galveston Subsidence District was created in 1975. Its mission is to control subsidence and manage groundwater resources. They regulate groundwater withdrawal and promote conservation. They also cooperate with surface water suppliers.
The United States Geological Survey maintains 14 borehole extensometers in the Houston-Galveston area. The Southwest Extensometer is one of these crucial instruments. It measures compaction from different aquifers.
Think of the Southwest Extensometer as a highly sensitive measuring device. It is anchored deep within the earth. It detects even the slightest vertical displacements in the ground. More specifically it measures compaction of the interval between the bottom of the extensometer and the land surface.
The Southwest Extensometer works like this. It is drilled to a significant depth, sometimes thousands of feet. It is anchored in stable strata using a cement plug. The extensometer uses analog recorders. These recorders continuously measure the change between a reference point on the inner pipe and the land-surface elevation. This provides data about the compaction of specific aquifers.
GPS stations are also installed at extensometers. Some are placed on the inner pipe to monitor the movement of the cement plug. Others monitor land-surface movement around the extensometer.
Data from the Southwest Extensometer has been invaluable. It helps us understand the relationship between aquifer depressurization and aquifer compaction.
While extensometers provide critical data their installation is expensive. The HGSD has moved toward more cost-effective methods such as GPS technology. These technologies also provide accurate land subsidence measurements.
The Southwest Extensometer stands as a testament to our ongoing effort. It shows our dedication to understanding and managing our precious groundwater resources.