Repeat Gravity Measurements at the Tongonan Geothermal Field, North Central Leyte, Philippines

Abstract

Leyte Geothermal Production Field (LGPF) in North Central Leyte is the largest geothermal area in the Philippines. It is comprised by two independent hydrothermal systems: the Tongonan Geothermal Field (TGF) in the north and the Mahanagdong Geothermal Field (MGF) in the south.

The commissioning of additional power plants beginning 1996 to harness the full energy potential of the area led to a massive mass extraction from the fieldÆs reservoir, which to date has incurred a cumulative net mass loss of 291 Mtons in TGF. Microgravity measurements, in tandem with precise leveling survey were conducted over the area to determine the reservoirÆs response to exploitation by detecting the minute exploitation-induced changes in gravity over time. The most recent measurements were conducted in 2003.

The greater than 100 ?gal decrease in gravity values since 1997 was recorded at TGF coinciding with the production field. This was accompanied by a corresponding ground subsidence, which had a maximum amplitude of about 18 cm, likewise occurring within the production sector of the geothermal field.

The measured decreases in gravity were associated with the mass loss from the reservoir. The data correlated well with the pressure drawdown of about 4-4.5 Mpa presently experienced over TGF.