Coil array for a high resolution induction logging tool and method of logging an earth formation

DETAILED DESCRIPTION It is therefore one object of the present invention to provide an improved dual induction tool.
It is another object of the present invention to provide an improved dual induction tool having high vertical resolution.
It is another object of the present invention to provide a new induction tool coil array design wherein different coils of the array control the vertical and radial focusing, so such an array may possess both sharp vertical resolution and deep radial penetration without the compromises present in known arrays.
An induction tool is provided which includes a coil array having a first plurality of transmitter coils and a second plurality of receiver coils.
The transmitter and receiver coils are symmetrically arranged on an insulated sonde about a selected point on the insulated sonde.
The induction tool further may include an oscillator circuit connected to the transmitter coils to provide a selected frequency transmission signal to the transmitter coils.
A receiver circuit is preferably connected to the receiver coils for receiving signals from the receiver coils.
In one embodiment of the present invention a centrally located transmitter coil assembly may be provided.
The transmitter coil assembly is positioned symmetrically about a selected point on an insulated body.
A plurality of receiver coils may be connected in series in this embodiment and have the same sense.
These receiver coils are positioned symmetrically about the selected point on the insulated body and the transmitter coil assembly.
An oscillator circuit is utilized to provide a selected frequency transmission signal to the transmitter coil assembly and a receive: circuit is similarly connected to the receiver coils.
In this embodiment the oscillator circuit also provides a phase reference signal to the receiver circuit.
The receiver circuit in turn detects an in-phase signal and out-of-phase signal.
Each of the receiver coils may further include two bucking coils that are symmetrically positioned about a respective receiver coil and, in one depicted embodiment, the bucking coils are connected in series and wound in opposite sense to the respective receiver coil