Deformation and failure of rock samples probed by T1 and T2 relaxation.
The objective of the study was to pinpoint the effect of stress induced rock matrix alterations on NMR-wireline-log measurements by means of laboratory T1 and T2 relaxation time measurements. The research activities were subdivided into two major parts: NMR relaxation measurements on a brine saturated outcrop sandstone (Red Wildmoor Sandstone) during uniaxial compressional tests and NMR relaxation measurements on artificial sandstone samples prepared with defined crack patterns. T1-measurements performed on Red Wildmoor samples during compaction showed a decrease in the mean relaxation rate 1/exp<ln (T1) up to a point assumed to characterize the yield strength of the rock. The rate decreased most probably as a result of the compaction of micropores. This trend was reversed when the rock failed. The relaxation rate increased again, most probably through the generation of fresh mineral surfaces at broken cementations or grain contacts. T2-relaxation was measured on artificial sandstone samples using CPMG pulse sequences with echo times between 175 micros and 1400 micros. Samples with larger crack volumes and crack orientation orthogonal to the B0-field showed consistently smaller exp<ln (T2) than the samples without any cracks. Further calculations indicate that internal magnetic field gradients were largest for samples having a larger number of cracks and crack orientations orthogonal to the B0-field.
Probing pore characteristics of deformed chalk by NMR relaxation
Abstract 1H Nuclear Magnetic Resonance (NMR) relaxation time was applied to investigate pores characteristic of chalk cores which were subjected to prior mechanical loading. The NMR relaxation responses of the cores loaded in elastic region (before yield) and in plastic region (after yield) were compared. T1,2 relaxation measurements performed on fully water saturated cores resolved a typical pore size corresponding to large pore body, which is affected by stress application particularly beyond the yield point. The 1/T1,2 relaxation rate also appeared to correlate with the volumetric strain. The NMR signal from the partially saturated cores (Sw 13 % ), however, showed no respectable change despite being loaded beyond the yield point. The T2 signal was dominated by two relaxation rates, which we argue corresponds to the surface water film ( ∼ 10 nm ) and micropores ( ∼ 0.1 μ m ) . The point that T1,2 NMR relaxation showed no significant change on partially saturated cores loaded beyond the yield point may indicate that the structure of water film remains unchanged despite progressive decay of pores structure during pore collapse failures.
What Happened to Suicide Bombings in Israel? Insights from a Terror Stock Model
An analysis of three years of suicide bombing data in Israel reveals an increase in such attacks through March 2002 followed by a steep decline through the end of 2003. The authors propose a terror-stock model that treats the suicide bombing attack rate as a function of the number of terrorists available to plan and execute suicide bombings. The intent of Israeli tactics such as targeted killings and preemptive arrests is to reduce the capacity of terror organizations to commit attacks. When fit to the data, this model suggests that the targeted killing of terror suspects sparks estimated recruitment to the terror stock that increases rather than decreases the rate of suicide bombings. Surprisingly, only the deaths of suspected terrorists, and not Palestinian civilians, are associated with such estimated recruitment. Although Israeli actions have reduced the rate of suicide bombings over time, it is preventive arrests rather than targeted killings that seem more responsible for this outcome.