| Title | 3D Imaging Of High Resolution Airborne Magnetic Data Over Mokai Geothermal Field, New Zealand |
|---|---|
| Authors | Soengkono, Supri |
| Year | 2014 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Airborne magnetic survey, reduction to the pole (RTP), progressive upward continuation, |
| Abstract | The first magnetic study of the Mokai Geothermal Field was made back in 1985, based on a low resolution airborne magnetic survey carried out in 1983 at elevations of about 760 m asl by the Geothermal Institute (University of Auckland). More recently, the Mokai area was included in a detailed airborne magnetic survey over the central Taupo Volcanic Zone made in 2005 by the gold exploration company Glass Earth NZ Ltd. The 2005 survey was conducted at 60 m ground clearance along E-W main flight lines separated by 150 m spacing. A technique for 3D imaging of high resolution airborne magnetic data was developed by the author using a combination of upward and downward continuations and verified by applying it to theoretical synthetic anomalies computed for a 3D source model. The aim of the technique is to image, from actual observed anomalies, the distribution of magnetisation at different depths. In the previous 1985 study, the Mokai reservoir was shown to be associated with a simple 3D body of homogeneous zero magnetisation with flat top and base and vertical edges, located between 400 m and 900 m depths (between about +0 m RL and -500 m RL). Inside the body all ferromagnetic minerals (magnetite and titanomagnetite) are inferred to have been completely altered by hydrothermal fluids to non-magnetic sulphide minerals (mainly pyrite). The 3D imaging of the 2005 high resolution airborne magnetic data from this study verified the broad figure of the 1985 3D model. However, the 3D imaging of the 2005 data also reveals details of the hydrothermal demagnetisation associated with Mokai geothermal activity not shown in the 1985 forward modeling result. Two areas of steam heated thermal manifestations appear to be associated with near surface extensions of the hydrothermally demagnetised rocks. Some zones of low magnetisation detected by the 3D imaging of the 2005 high resolution airborne magnetic survey could be associated with NNE outflow of geothermal fluids from the Mokai field revealed by the result of previous DC-resistivity survey. |