Abstract

Magneto telluric and Schlumberger resistivity surveys have been done by PERTAMINA on Lahendong geothermal area in North Sulawesi region, during 1982 – 1983 period.

A total of 201 magneto telluric soundings and 291 Schlumberger profilings and 44 soundings were measured.

The magneto telluric sounding were measured with a wide frequency band from 128 Hertz up to 0.005 Hertz and the Schlumberger soundings were made with AB/2 maximum 2000 m.

From the magneto telluric and Schlumberger qualitative and quantitative interpretation results, a low resistivity of less than 10 Ohm-meter can be found in several parts of the area, likely to be related to strong to medium hydrothermal alterations.

The interpretation of Schlumberger resistivity measurements indicated from iso-apparent resistivity maps showed several anomalies in Lahendong, Waloan and Rurukan in the northern part and Tompaso area in the southern part. Most of these anomalies relate to near surface alteration zone. From one dimensional interpretation of Rho-invariant diagram of the MT measurement it can be assumed that the possible reservoir indicated by resistive basement lies below the conductive (alteration) zone.

And from three dimensional structure map constructed from Magneto telluric resistivity sections one can see clearly the distribution of the top of possible geothermal reservoir and its related depth, and some NW–SE trending faults.