Antenna and wear plates for borehole logging apparatus

摘要

An apparatus is disclosed for investigating nuclear magnetic resonance properties of earth formations traversed by a borehole, including a logging device adapted for longitudinal movement in the borehole. The device includes a first subsystem for producing a static magnetic field in the formations and a second subsystem for transmitting electromagnetic energy into the formations and for detecting electromagnetic energy from the formations. The second subsystem comprises an antenna assembly which includes an open-ended shell formed of a magnetically permeable material. An open-ended electrically conductive body is disposed within the shell and is spaced therefrom. A medium whose acoustic impedance is substantially different than the acoustic impedance of the shell and the body is disposed between the shell and the body. An electrically conductive probe is disposed in the body, and an electrically insulating magnetically permeable loading material disposed in the body. The shell is formed of a material having a magnetic permeability that is high enough to render the magnetic reluctance of the loading material several times the magnetic reluctance of the shell. The body is formed of a metal having thickness that is at least several skin depths in said metal at the frequency of the electromagnetic energy and is less than the minimum thickness that would support an acoustic resonance mode in its thickness direction at the frequency of the electromagnetic energy. The apparatus provides a shunt path for the static magnetic field in the region of the antenna that would otherwise have a deleterious effect on the loading material of the antenna and on antenna operation. The apparatus also minimizes undesirable ringing of the antenna.