| Title | Geophysical Techniques for Shallow Hot Water Exploration: Lessons from Some NZ Case Studies |
|---|---|
| Authors | C. J Bromley, S. Soengkono, R. Reeves & S. Bennie |
| Year | 2006 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | |
| Abstract | In New Zealand, the search for hot water resources at shallow depths for the purposes of direct heating applications has a long history. Geophysical methods, particularly resistivity, are frequently called upon to provide a robust exploration target for drill hole siting (often in conjunction with less scientifically robust methods such as water divining). We have compared the results of recent resistivity trials, conducted in a variety of settings, and using a variety of DC and electromagnetic (EM) methods, with the more traditional methods used in the past. These trials have been conducted at Whitford and Naike in the Auckland region; Miranda and Pipiroa in the Hauraki Plains, Hot-water beach at Coromandel; Awakeri, Mokoia, and Horohoro in the Taupo Volcanic Zone (TVZ); Ohakune to the SW of TVZ; and at Hanmer Springs in the South Island. The results of these trials show that temperature measurements at shallow depth (ÑT6m) may, in some places, help detect shallow hot water resources. Detailed gravity measurements can be useful, but are usually ineffective at directly detecting fracture zones that channel upflow of hot water into shallow aquifers. Electrical resistivity investigations using the electromagnetic (EM) methods (particularly CSAMT) have some advantages in terms of efficiency of data collection and improved resolution of subsurface resistivity structure to about 300m depth, but are highly susceptible to cultural electrical noise sources. DC resistivity methods (especially tensor-gradient and Schlumberger array) have some advantages in such environments, but they are usually less efficient at resolving narrow target structures. All the resistivity methods suffer from distortions caused by earthed metal structures, and from penetration depth limitations in regions containing highly conductive clays. Interpretation of resistivity anomalies is site specific. In some settings a narrow resistive structure may signal a fault-controlled zone of silicification within an otherwise impermeable clay-rich layer. In other settings, a permeable fault zone may contain conductive clays and mineralized hot fluids within an otherwise resistive host rock. Where structure is complicated (3-D) a full tensor approach may be advisable, despite the added cost, because of the risk that distorted scalar resistivity results may be misinterpreted. |