Time Domain Electromagnetic Resistivity Measurements at Ngatamariki Geothermal Field, New Zealand

Abstract

The applicability of the time-domain electromagnetic method for determining detailed resistivity structure within a geothermal field was tested at Ngatamariki. The transient electric field vectors, which result from injection of low-frequency square-wave signals into the ground through three remote current sources, could be determined reliably for times between 0.02 and 3.3 s .after each step in signal. After making small corrections for static shift, apparent resistivity pseudosections along the two measurement lines show smoot;h variations of resistivity from site to site. Over most of the field the images consistently show a three-layer resistivity structure with a conductive middle layer (3 - 10 Szm) representing the conductive upper part of the thermal reservoir. A deep-seated region of low resistivity in the north-west of the field may indicate a conductive structure at about 1 km associated with a deeper diorite intrusion. Measurements sited closer than about 100 m to drillholes appear to have been disturbed by metallic casing in the holes. A change in resistivity structure in the east of the field may indicate a major geological or hydrothermal boundary.