| Abstract |
Close-spaced resistivity measurements along ten traverse lines crossing the resistivity boundary of the Ohaaki Geothermal Field were first measured in 1975 and remeasured in 1992. The 1992 resistivity profiles were similar in shape to the original ones. On both occasions very sharp resistivity boundaries were delineated along tlie southern and south-western edges of tlie Field where apparent resistivity rises sharply over a horizontal distance of a few hundred nietres from 2-5 m on the inside of the Field to 20-50 m on the outside. On two of southern lines the resistivity boundary appears to have moved outwards by about 100 m, and on two others the outward movement may be as niuch as 20 m,but this is of the same order as the survey resolution. Over the 17 year interval apparent resistivity values have dropped slightly at most measurement sites. of this change (up to 10 percent) may be due to calibration errors and measurement difficulties. The decrease is pronounced on the inside of the Field where apparent resistivities have declined by up to 30 - 40 percent. This is thought to be caused mainly by the disturbing effects of the new drillholes and steam pipes that have been installed since 1975, and by changes in the watertable. In the north-west of the Field, movement of the resistivity boundary could not be clearly detected because of reduced resolution of the survey over this region and because of large disturbances to the signal buried conductors such as the earth at tlie Power Station, pipes and drillhole casings. |