Long-term temperature monitoring in a borehole drilled into the Nojima Fault, southwest Japan

Authors
Citation
M. Yamano et S. Goto, Long-term temperature monitoring in a borehole drilled into the Nojima Fault, southwest Japan, ISL ARC, 10(3-4), 2001, pp. 326-335
Citations number
26
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
ISLAND ARC
ISSN journal
1038-4871 → ACNP
Volume
10
Issue
3-4
Year of publication
2001
Pages
326 - 335
Database
ISI
SICI code
1038-4871(200109/12)10:3-4<326:LTMIAB>2.0.ZU;2-Y
Abstract
Long-term monitoring of temperature distribution in an active fault zone wa s carried out using the optical fiber temperature-sensing technique. An opt ical fiber cable was installed in a borehole drilled into the Nojima Fault in Awaji Island, south-west Japan, and the temperature profile to a depth o f 1460 m had been measured for 2.5 years (July 1997-January 2000). Although the obtained temperature records showed small temporal variations due to d rifts of the measurement system all along the cable, local temperature anom alies were detected at two depths. One at around 80 m seems to correspond t o a fracture zone and may be attributed to groundwater flow in the fracture zone. This anomaly had been stable throughout the monitoring period, where as the other anomaly at around 500 m was a transient one. The water level i n the borehole could be estimated from the diurnal temperature variations i n the uppermost part of the borehole and may provide information on the hyd rological characteristics of the fault zone, which is connected to the bore hole through perforations on the casing pipe. Except for these minor variat ions, the temperature profile had been very stable for 2.5 years. The condu ctive heat flow calculated from this profile and the thermal conductivity m easured on core samples increases with depth, probably resulting from error s in thermal conductivity due to sampling problems and/or from advective he at transfer by regional groundwater flow. Assuming that the middle part of the borehole (less fractured granite layer) is least affected by these fact ors, heat flow at this site is estimated to be approximately 70 mW/m(2).