Non-linear tides observed with the superconducting gravimeter

Abstract

SUMMARY A four-year record from the superconducting gravimeter at Cantley, Quebec, is examined for non-linear tidal effects. A linear response is first determined from an adaptation of the response method used in the tidal analysis of sea-level. Residuals left by the linear response clearly show anomalies of a few tens of ngals (1 ngal= 10-l' ms-'). These residuals are most evident in the semi-diurnal band, but the diurnal, ter-diurnal and quart-diurnal bands are also affected. A subsequent analysis shows that these are most likely the loading effects of shallow-water non-linear tides, principally in the Bay of Fundy and the east coast of North America. The loading effect of global degree-three diurnal and semi-diurnal tides is also interpreted. The amplitude of the latter tides in the open ocean is a few millimetres, so the superconducting gravimeter is sensing ocean tides at a level surpassing that of most standard tide gauges, and of satellite altimetry. The linear response analysis, and the identification and removal of the non-linear tides, provides the clearest look to date at the tide-free spectrum. It is shown that any simple non-linear response that doubles the frequency must be at least 65 dB below the linear response, if the source of the non-linearity is in the solid Earth, and 82 dB below the linear response, if the source of the non-linearity is in the instrument. If the observed non-linear response is entirely due to loading by non-linear ocean tides, as seems likely, then these figures are 76 dB and 93 dB, respectively.