Further Statistical Analysis of Direct Subsoil Temperature Data (Including Residuals Analysis) at the Pailas Geothermal Field and in the Rincon de la Vieja National Park, Guanacaste, Costa Rica

Abstract

From June 2008 to September 2009, a field investigation for a Master's thesis in Natural Resource Management was carried out to identify the existence of subsoil thermal anomalies that may be correlated with anomalous heat flow along local faults at the Pailas Geothermal Field, located on the Pacific slope of the Quaternary Rinc¨®n de la Vieja volcanic complex in northwestern Costa Rica. A total of 240 direct subsoil temperature measurements were made at fifty-six one-inch diameter access holes equally spaced on a one-kilometer grid and drilled to depths ranging from 47 to 167 cm in iso-hyperthermal entisols and thermal to iso-hyperthermal inceptisols. This study covers the area that directly impacted by the operation of Pailas 1, a 35 MWe binary rankine-cycle geothermal plant inaugurated in July 2011, part of the north bordering Rinc¨®n de la Vieja National Park and an area extending towards the northwest of the currently developed geothermal field. Mean annual subsoil temperatures have been reduced to their respective Z values for standard deviation analysis and residuals from multiple regression analyses on data collected at the end of the dry season (¦Á=0.05) are studied to try to pinpoint outliers that significantly deviate from the estimated mean value and may represent a deep-seated geothermal anomaly. There appears to be a north-south trend of significantly higher residuals at higher elevations and an east-west trend at lower elevations.