4 research outputs found
Passive Seismic Tomography Using Induced Seismicity at a Petroleum Field in Oman
A borehole network consisting of 5 monitoring wells was used to monitor the induced
seismicity at a producing petroleum field for a period of about 11 months. Nearly 5400
microseismic events were analyzed and utilized in imaging the reservoir based on a new doubledifference
(DD) seismic tomography. The DD tomography method simultaneously solves for
event locations and Vp, Vs, and Vp/Vs models using absolute and differential P, S and S-P
arrival times. Microseismicity in the field was primarily caused by compaction of the reservoir in
and above the gas bearing formation and was distributed along the two major northeastsouthwest
(NE-SW) faults in the field. The model resolution analysis based on the checkerboard
test and the resolution matrix showed that the central part of the model was relatively well
resolved for the depth range of 0.7 to 1.1 km. Clear velocity contrasts were imaged across most
parts of the two NE-SW faults. Vp/Vs ratio estimations from the tomographic inversion were
low (<1.75) in the shallow depth range, likely due to lithology and gas content, whereas they
were large (>1.75) in the deeper part of the model, likely due to fluid saturated formation. In this
study seismic tomography showed a great potential for reservoir imaging and property estimation
using induced seismicity.Petroleum Development Oma
Self-Organisation and Dissipation in Real and Synthetic Earthquake Populations
Energy released from the Earth’s crust in the form of earthquakes commonly follows a powerlaw
gamma type probability distribution. This spontaneous organisation is in apparent contradiction
to the second law of thermodynamics that states that a system should naturally evolve
to a state of maximum disorder or entropy. However, developments in the field of modern
thermodynamics suggest that some systems can undergo organisation locally, at the expense
of increasing disorganisation (or entropy) globally through a process of entropy production.
The primary aim of this thesis is to investigate self-organisation in the Earth’s seismogenic
lithosphere as a driven, far-from-equilibrium, self-organising ‘dissipative structure’ in a very
near critical steady-state and the underlying general mechanisms involved. The secondary aim
is to test in more detail the applicability of the Bak, Tang and Wiesenfeld (BTW) model of
Self-Organised Criticality (SOC) in describing Earth’s seismicity. This is done by: 1. Mathematical
derivation of analytical solutions for system energy and entropy using the tools of
equilibrium statistical mechanics; 2. The study of conservative and non-conservative versions
of the BTW numerical model and 3. Analysis of temporal and spatial properties of earthquake
data from the Harvard Centroid Moment Tensor catalogue and the Global Heat Flow Database.
The modified gamma distribution predicts analytically that entropy S is related to the energy
probability distribution scaling exponent B and the expectation of the logarithm of seismic
energy hlnEi in the form of the gamma entropy equation S » BhlnEi. This solution is con-
firmed for both numerical model results and real earthquake data. Phase diagrams of B vs.
hlnEi suggest that the universality in B need not be maintained for a system to remain critical
provided there is a corresponding change in hlnEi and S. The power-law systems examined
are different from equilibrium systems since the critical points do not occur at global maximum
entropy. For the dissipative BTW model at a steady-state, the externally radiated energy
follows out-of-equilibrium power-law gamma type statistics, but, the internal energy has two
icharacteristics that are indicative of equilibrium systems; a Gaussian type energy probability
distribution and a Brownian noise power-spectrum for the internal energy fluctuations. This
suggests an observer dependency in assessing criticality. The internal and external entropies
calculated for the model are negatively correlated suggesting that driven systems self-organise
at the expense of increasing entropy globally through a process of dissipation. A power-law
dependency of mean radiated energy hEi on dissipation 1¡® is confirmed for a locally driven
dissipative system in the form hEi » (1¡®)¡0:975. The BTW model shows spatial heterogeneity
whilst maintaining universality in contradiction to previous assumptions. The quantitative
analysis of real data reveals that earthquakes are more predictable spatially then temporally.
Regionalisation using the Flinn-Engdahl classification shows that mid-ocean ridges are more
organised (lower entropy) than subduction zones. A regional study of three different scaling
exponents suggests that universality in earthquake scaling is violated, in contradiction to the
original model of SOC. A model of self-organised sub-criticality (SOSC) is proposed as an
alternative model for Earth seismicity. Overall, the results suggest that the tools of equilibrium
thermodynamics can be applied to a steady-state far-from-equilibrium system such as the
Earth’s seismogenic lithosphere, and that the resulting self-organisation occurs at the expense
of maximising dissipation and hence entropy production