Tensor time domain electromagnetic resistivity measurements at Ngatamariki geothermal field, New Zealand

Abstract

Experimental measurements in the Ngatamariki geothermal field, North Island, New Zealand were made to test the applicability of the time domain electromagnetic method for detailed investigation of the resistivity structure within a geothermal field. Low-frequency square wave signals were transmitted through three grounded bipole current sources sited about 8 km from the measurement lines. Despite high levels of electrical noise, transient electric field vectors could be determined reliably for times between 0.02 and 3.3 s after each step in the source current. Instantaneous apparent resistivity tensors were then calculated. 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 northwest 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.

Introduction

Since its introduction in the 1960s resistivity surveying has proved very useful for delineating the geothermal fields in the Taupo Volcanic Zone (TVZ) of New Zealand. The highly conductive nature of the hot water and minerals that comprise the shallower parts of the geothermal reservoirs has proved a suitable target for shallow resistivity surveying (DC resistivity), allowing the conductive reservoirs to be delineated against high-resistivity surroundings (Bibby, 1988).

However use of geophysical methods for determining details of geological structure within the fields has proved more elusive. Traditional DC resistivity methods as well as gravity and magnetic surveying can at best determine only broad-scale features within a geothermal field. Seismic reflection surveys have also proved of limited use due to strong attenuation and strong reverberation effects in the near-surface layers of unconsolidated volcaniclastics (Bannister and Melhuish, 1997). Such short-comings of geophysical methods in detailed exploration of geothermal fields contrast with the record of the oil industry in exploring petroleum prospects. Determination of structure in the deeper parts (>1 km) of most oil fields relies heavily on obtaining good quality images from seismic reflection surveys.

While DC methods are best used to provide a map view of the prospect, information on the variation of resistivity with depth beneath each measurement site can also be derived from the transient behaviour of the measured electric fields. Here, we will derive apparent resistivity tensors from these transients using the time domain electromagnetic (TDEM) method, which is sometimes referred to as the LOTEM method (long offset transient electromagnetic; Strack, 1992). This is an extension of the multiple-source bipole–dipole method used for DC resistivity surveys. Application of apparent resistivity tensor analysis to the transient EM data (Caldwell and Bibby, 1998) facilitates the visualisation of transient data using distance–time resistivity sections (‘pseudosections’) analogous to seismic reflection sections. These provide representations of the subsurface in more detail than can be done with DC resistivity surveying.

An earlier experiment with the TDEM technique made at Wairakei geothermal field (Caldwell et al., 1999) was able to delineate the northern boundary of the field and deduce some details of its internal structure. This paper discusses further experiments made in March 2000 with the TDEM method in the Ngatamariki geothermal field (Fig. 1). The intention in this work is to adopt some of the attributes of seismic reflection surveying by making close-spaced measurements along lines across the field and to test the practicality of obtaining images that are coherent from site to site and can be better correlated to the underlying hydrothermal structures than has been achieved with traditional DC resistivity methods. The emphasis is on investigating the internal structure of the field rather than on locating its boundaries, as was the intent of most earlier DC resistivity surveys.