Publication Date

2018-07-18

Availability

Open access

Embargo Period

2018-07-18

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine Geology and Geophysics (Marine)

Date of Defense

2018-05-29

First Committee Member

Keir Becker

Second Committee Member

Christopher G. A. Harrison

Third Committee Member

Guoqing Lin

Fourth Committee Member

John C. Van Leer

Abstract

Implementations of three models were used to constrain permeability and vertical bulk modulus of oceanic crust using pressure data collected from CORKs at ODP/IODP Holes 1024C, 1025C, 1026B, 1027C, and 504B. The models are based on a one-dimensional half-space model, which interprets a given sinusoidal signal within formation as a combination of two signals produced by ocean tidal loading: an elastic signal at the measurement site that is in-phase with tidal loading, and a diffusive signal that travels through the formation to the instrument site. The aforementioned sites were chosen due to their thick sedimentation: the sediment layer is too thick to allow a diffusive signal through, so diffusive signals can only be generated at thinly sedimented or exposed basement and be transmitted laterally through basement. Harmonic analyses were applied to seafloor and formation pressure data gathered from the CORKs to determine the tidal loading signal at the seafloor and the tidal signal within the formation fluid; the signals analyzed were at the S2, M2, K1, and O1 frequencies. Two implementations of the one-dimensional half-space model were used: a brute force method, and a least squares method. These were used compared to each other and to previous studies; the brute force method was found to be better overall, though the least squares method agreed with the brute force method in constraining the elastic signal. A two-dimensional isotropic model was used to constrain the “bulk” or “isotropic equivalent” values of permeability; a two-dimensional anisotropic model was used to constrain the ridge-parallel and ridge-perpendicular components of permeability. All three models show a decrease in permeability with increasing crustal age, with a drop of one to two orders of magnitude ([10^-11]-[10^-10] m^2 to [10^-12]-[10^-11] m^2) from roughly 1 Ma to 6 Ma. Overall, permeability showed a faster decrease with diurnal components versus semidiurnal components. The anisotropic model suggests that the oceanic crust has greater permeability along the ridge-parallel direction, and that the ratio of the ridge-parallel to ridge-perpendicular components decreases from over an order of magnitude at 1 Ma to a third of an order of magnitude at 6 Ma. While the ridge-parallel components of permeability roughly decreased with increasing crustal age as previously mentioned, the ridge-perpendicular components decreased by roughly a quarter of an order of magnitude in the same span of time. For the vertical bulk modulus, the constraints found generally agreed with previous studies; however, the ranges were somewhat too large for usefulness. The aforementioned models were also used to analyze pressure data from Hole 1025C before and after an Mw 4.3 seismic event that occurred on October 4, 1997 roughly 14 kilometers ENE of Site 1025. The pressure data was broken up into 29-day intervals; from there harmonic analyses were done on each 29-day dataset, and the three models were used to constrain the crustal properties. For all three models permeability increased immediately after the seismic event, and proceeded to decay to pre-seismic levels over roughly a year and a half. This is interpreted to be due to the sealing of fluid flow pathways by deposition that had been opened by the seismic event.

Keywords

crustal hydrogeology; marine geophysics; marine observatories; tidal analysis; anisotropic permeability; bulk permeability

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