Chris J. Bromley

Steam cloud characteristics and heat output of fumaroles

Abstract The heat transferred to the atmosphere by fumarole clouds is linearly proportional to the cloud area that is visible in vertical air-photos. Using measured outputs of a few accessible fumaroles, the total heat output of all fumaroles in a fumarole field can be assessed from the total area of the steam clouds. Normalised heat outputs, Q j , and normalised cloud areas, A j , were used as variables. Two surveys of the large Karapiti fumarole field (Wairakei, New Zealand) show that the total heat discharged by fumaroles with cloud areas >10 m 2 was 99 MW and 88 MW in 1999 and 2000, respectively. The output of vents with smaller cloud areas has to be obtained from ground surveys. The method also gives a representative (order of magnitude) estimate of the total heat discharged by fumarole clouds if measurements from a single, large fumarole can be used in conjunction with a recent air-photo.

Geophysical techniques for low enthalpy geothermal exploration in New Zealand

Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore usinggeophysicaltechniques,mainlybecausethewarmwatercreatesaninsufficientphysicalchangefromthehostrockstobe easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success. Asimpleanddirectmethodfortheexplorationofwarmwatersystemsisshallowtemperaturemeasurements.InsomeNew Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water. ThegravitymethodwasutilisedasastructuraltechniquefortheexplorationofsomewarmwatersystemsinNewZealand. Ourcasestudiesshowthetechniquecanbeusefulinidentifyingbasementdepthsandtracingfaultsystemsassociatedwiththe occurrence of hot springs. Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extentofshallowwarmwatersthataremoreelectricallyconductivethanthesurroundingcoldgroundwater.Groundresistivity investigationsusingtheelectromagnetic(EM)techniquesofaudiomagnetotellurics(AMTorshallowMT),controlledsource audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner.