Quantifying motional dynamics in nuclear magnetic resonance logging.

The motional dynamics of nuclear magnetic resonance (NMR) logging tools can significantly influence the measurement performance of such tools. NMR logging is used for geophysical evaluation in geological environments, primarily quantifying formation porosity and fluid volumes, as well as providing a qualitative estimation of permeability. NMR logging tools are conveyed via two main mechanisms; wireline logging and logging while drilling (LWD). We conduct detailed simulations to quantify the impact of tool motion on NMR measurements during logging. This involves conducting electromagnetic simulations which quantify the magnetic fields generated by a logging tool, and subsequently introducing motion profiles within the relevant spin dynamic calculations. This enables tool motional dynamics to be imposed on the signal acquisition. Several movement profiles are considered: linear axial movement to replicate wireline logging tool motion, as well as axial harmonic and lateral harmonic movement to simulate the shocks and vibrations experienced during logging while drilling. Lateral motion is observed to cause a greater degree of signal attenuation relative to axial motion due to the cylindrical shape of the excited volume. The magnitude of motion (e.g. the velocity of linear motion or the amplitude of harmonic motion) is demonstrated to increase the severity of signal attenuation, as expected. However, the frequency of harmonic motion demonstrates a more complex effect on the measured signal. The harmonic interaction between the motion frequency and measurement frequency (determined by the echo spacing) can cause wave interference which results in enhanced or diminished signal attenuation. Finally, we demonstrate that reducing both the magnetic field gradient as well as the echo spacing reduce the degree of signal attenuation observed during measurement. The results presented in this work demonstrate how the optimisation of key design parameters can be used to control the sensitivity of NMR logging tools towards motion.

Quantitative velocity distributions via nuclear magnetic resonance flow metering.

We demonstrate the use of Tikhonov regularisation as a data inversion technique to determine the velocity distributions of flowing liquid streams. Regularisation is applied to the signal produced by a nuclear magnetic resonance (NMR) flow measurement system consisting of a pre-polarising permanent magnet located upstream of an Earth's magnetic field NMR detection coil. A simple free induction decay (FID) NMR signal is measured for the flowing stream in what is effectively a 'time-of-flight' measurement. The FID signal is then modelled as a function of fluid velocity and acquisition time, enabling determination of the velocity probability distributions via regularisation. The mean values of these velocity distributions were successfully validated against in-line rotameters. The ability to quantify multi-modal velocity distributions was also demonstrated using a two-pipe system.

Sudden cardiac death in the young: a 1-year post-mortem analysis in the Republic of Ireland.

AIMS: A retrospective post-mortem analysis was undertaken to determine the aetiology of sudden cardiac death (SCD) amongst individuals aged

Changing patterns in the incidence of Rocky Mountain Spotted Fever on Long Island (1971-1976).

Clinical data from 124 Rocky Mountain Spotted Fever (RMSF) cases submitted by Long Island physicians and hospitals were compiled for the six-year period 1971-1976. This information was subsequently compared to two parameters: first, the geographic distribution of human cases and rickettsia-infected Dermacentor variabilis (Say), and, secondly, the human population dynamics in Long Island towns since 1950. Despite an overall decline in the rate of suburbanization and population growth, cases of RMSF have increased significantly not only in eastern towns where RMSF has been endemic, but also in well-established western towns where recent population increases have been negligible. Phenomena associated with suburbanization, primarily the association of domestic and sylvan animals and their exposure to infected vector populations, may be instrumental in explaining the increased transmission of RMSF.