| Abstract |
Close-spaced resistivity measurements along ten traverse lines crossing the resistivity boundary of the Ohaaki Geothermal Field, New Zealand, 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 the southern and southwestern edges of the field where apparent resistivity rises sharply over a horizontal distance of a few hundred metres from 2-5 ohm m on the inside of the field to 20-50 ohm m on the outside. On two of the southern lines the resistivity boundary appears to have moved outwards by about 100 m, which may be caused by southward movement of reinjected waste water from nearby drillholes. On the other southern lines the outward movement appears to be less than about 25 m, which is the limit of resolution of the survey. Over the 17 year interval apparent resistivity values have dropped slightly at most measurement sites. The decrease is more pronounced on the inside of the field boundary where apparent resistivities have declined by up to about 40 percent. Some of this decrease is attributed to reinjection of conductive waste water near the field boundary causing a drop in ground resistivity. Part of this change may be due to calibration errors and measurement difficulties, including the disturbing effects of the new drillholes, steam pipes and an earthing mat that have been installed since 1975. |