Recharge at Broadlands (Ohaaki) Field 1967- 1983 Determined from Repeat Gravity Measurements

Abstract

A revision of previous repeat gravity measurements, made in 1967 and 1974, shows that for the period (during which time drawdown tests were made) negative gravity differences of about to -1.2 amplitude occurred over all of the field. Simple suggests that these difference s were the result of a net loss of about 24-35 x kg, and hence there was little or no mass recharge. Data from a recent survey made in 1983, show that for the period (during which time there was a reduced amount of mass withdrawal) positive gravi ty differ e n c e s of about 0.4 amplitude occurred in most parts of the field. Modelling suggests these differences resulted from a net mass a in of about 11 x kg, and hence total natura? recharge was about 25 x kg. During both periods there was mass loss from a large area west of the field, the cause of which is unknown. Elsewhere, outside the field, t here have been no significant changes.