Repeated precise gravity measurements on Vancouver Island, British Columbia

Abstract

Over the past 3 years, repeated precise gravity measurements have been made on Vancouver Island, British Columbia, to test a practical field technique for precise gravimetric surveys and to monitor possible gravity changes associated with contemporary crustal deformation. In order to obtain a precision for measured gravity differences of a few microgals (1 μGal = 10 nm/s2), rigorous field survey procedures were followed and the operating characteristics and calibration nonlinearities of the LaCoste and Romberg model D gravimeters were taken into account. Instrument evaluations indicate that although the standard error of the mean of several gravity ties is 1.5–2.0 μGal for an individual instrument, the present achievable accuracy of the model D gravimeter for the measurement of temporal gravity changes is 3–4 μGal, provided the dial settings of previous surveys are recovered or both long and short wavelength nonlinearities in calibration can be specified. Semiannual surveys of the central Vancouver Island region indicate significant temporal variations over the past 3 years. Gravity changes as large as 60 μGal at sites adjacent to lake shorelines are caused by the mass effect of changing water levels and can be accounted for by simple model calculations. A gravity effect of seasonal changes in groundwater levels is not identifiable in the data, although easternmost network sites are underlain by porous sediments subject to seasonal water table fluctuations of 1 or 2 m which are expected to cause gravity variations of the order of 10μGal. A spatially coherent pattern of longer‐term gravity trends is indicated in the northern network after corrections have been made for known mass changes. These trends can be interpreted in terms of relative elevation changes of the order of 2 cm/a between Buttle Lake and the area to the northwest. Alternatively, a change in density caused by secular changes of water content in fracture zones or by compression through an accumulation of stress could also cause the observed gravity trends.