Nonlinear barometric response of borehole strainmeters and its interpretation

The response of strain to atmospheric pressure changes is studied for six borehole strainmeter sites in Tohoku district, Japan. The analysis is carried out by two methods: one is time domain filtering and the other is Fourier transform in frequency domain. Though the responses at Oga (OGA), Nibetsu (NIB), Honjo (HOJ), and Gojome (GJM) are frequency independent, the response at Tazawako (TAZ) clearly depends on frequency. This phenomenon is not expected for an elastic or viscoelastic medium, but is due to the movement of groundwater through the medium. We simulate the frequency dependence using a fluid-filled porous medium model (Bum., 1941). This model explains the frequency-dependent response at TAZ and gives the porosity of 22% and the permeability of 2.5×10^-7 cm/s beneath this station.