Tidally Induced Changes of Fluid Pressure, Geothermal Temperature and Rock Seismicity

This paper analyzes and summarizes the numerous types of variations of groundwater pressures (heads), mid-oceanic ridge borehole temperatures and microseismicity in response to tidal forcing in coastal aquifers and deep sea sediments. Tidally related data obtained from various sources were filtered and processed by means of classical harmonic analysis to extract the amplitudes of pressure and temperature fluctuations at different depths, as well as seismicity tilting angles. It is apparent that ocean tidal fluid pressure signals are transported vertically and damp with depth in coastal aquifers and in deep sea sediments. In contrast, the opposite is true for tidal signals triggered by the solid earth tide, as they increase with depth. The vertically transported ocean tidal fluid pressure induces a periodical temperature change in the borehole fluid whose amplitude damps with depth and interrupts the geothermal gradient normally measured in the deep sea sediment. Using the adiabatic state equation of temperature, pressure and salinity (P-S-T), we are able to demonstrate to certain extent that observed periodical temperature changes in the borehole fluid by means of the measured tidally induced fluid pressure changes at the open end of the borehole. A clearly tidal effect was also observed in the filtered seismicity tilting data. Overall, geological phenomena induced by tidal forcing are probably more prominent and complicated than has been realized so far and warrant further investigations. The last section of the paper discusses the analytical prediction of propagation of tidal pressure with depth considering the aquifer deformation. The analytical solution and its application to ODP hole 1025c show that one can predict the vertical pressure change in a well or ODP borehole fairly well with an analytical solution.