| Summary: | Gravity techniques have been used firstly to determine the sub-surface structure of the Guayabo caldera together with the related part of the Guanacaste Arc, and secondly to study the temporal evolution of the Miravalles Geothermal Field. Detailed density and stratigraphy studies of both the drilled and exposed geology have both complemented and constrained the caldera gravity models. A new caldera volume - erupted mass balance model is proposed that computes the buried, intra-caldera mass and volume parameters in terms of the mass deficiency commonly associated with silicic calderas. The 10 km diameter topographic expression of the Guayabo caldera comprises nested collapse structures, the boundaries of which are defined within the caldera by distinct gravity discontinuities. The modelled structure shows that the Guayabo caldera formed by multistage collapse across discrete sub-parallel sets of ring faults, rather than by chaotic collapse. The resultant overall structure resembles a funnel caldera, and it is possible that similar structures may be present but remain undetected in comparable calderas. The 200- 400 m collapse across the faulted topographic margins is far less than the previous estimate of 2400 m, where the entire drilled stratigraphy was thought to comprise the fill to the caldera. Most of the stratigraphy drilled on the eastern margin of the caldera pre-dates the formation of the caldera, contrary to the previous theories. Furthermore, the caldera volume far exceeds the volume of the Guayabo tuffs and post-tuff subsidence, possibly related to the build-up of the recent Miravalles stratoconeso n the caldera margin, is required to explain the imbalance. An important result of the revised caldera model is that the geothermal reservoir host rocks continue beneath the caldera margin, and therefore the volume of the reservoir may be greater than previous estimates. The 35 mGal gravity couple over the inner margin of the Nicaraguan depression is modelled as the near-surface outcrop of the Cretaceous ophiolitic basement, previously thought to be present at several km depth beneath the Guanacaste arc. Gravity data show that the extensive Plio-Pleistocene silicic pyroclastics of Guanacaste do not have detectable deepseated low-density plutonic equivalents. Repeat gravity studies at Miravalles identified 40 tGal fluctuations on a monthly time-scale, due to groundwater movement in the Guayabo caldera rather than geothermal-related sources. Experimental data have shown that LaCoste and Romberg gravimeters record repeatable gravity decreases of up to 700 pGal when subjected to vibration at specific harmonic frequencies. This accounts for the gravity decreases observed during well testing in 1988, that could not be explained by reasonable mass changes. Talring these points into consideration, a repeat gravity monitoring program was designed to both test and complement the future reservoir simulation models.
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