Temperature Gradient Map of the Hakkoda Geothermal Area

Abstract

There are many methods for estimating subsurface temperature for the geothermalassessment of a wide area of several tens kilometers by several tens kilometers, but the best way is to obtain data from boreholes directly. Ninety-nine borehole temperature log data in the Hakkoda geothermal area were gathered and a temperature gradient map was drawn. The data were obtained from many kinds of boreholes of geothermal and mineral exploration and hot spring prospecting. Background temperature gradient of the map ranges from 40°C/km to 60°C/km, and high temperature gradient regions over 100°C/km occur over the northern Hakkoda volcanoes and the Okiura and Aoni hot springs. The value of the background corresponds to the average temperature gradient of the backarc side of Japanese Islands. The map was compared with volcanic activities, geochemical and geophysical prospecting including magnetotelluric, gravity, and magnetic surveys. The map of temperature gradient reveals the subsurface temperature between the ground surface and several kilometers depth and is appropriate to assess geothermal resources and choose promising areas in a reconnaissance stage. Borehole data of hot springs without any description of temperature logs are useful only if their measured depths are sufficiently deep.