TIME DOMAIN ELECTROMAGNETIC RESISTIVITY MEASUREMENTS AT NGATAMARIKI GEOTHERMAL FIELD, NEW ZEALAND

摘要

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 smooth 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 Ωm) 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.