Keeping in mind the possibility of large \theta 13, which will be soon explored by reactor and accelerator experiments, I formulate a perturbation theory of neutrino oscillation under the ansatz s132 \simeq \Delta m221 / \Delta m231\equiv \epsilon \simeq 0.03, which is comparable to the Chooz limit. Under the framework, I derive the perturbative formula of the \nu e appearance probability valid to the order of \epsilon 2 in which effects of arbitrary matter density profile is taken into account. I use the formula to analyze problem of possible obstruction to detecting lepton CP violation by effects of asymmetry in matter density profile. Though the asymmetry could be large for neutrino trajectories which traverse both continental and sea crust, its effect on obscuring CP violation measurement is found to be quite small.
PACS numbers: 14.60.Pq, 14.60.Lm, 91.35.–x
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