Compact magnetic sensor for multi-component induction and micro-resistivity measurements

A method for logging a formation surrounding an open hole, comprising:

• providing a winding;
• providing a core upon which the winding is wound, the core having a first end and a second end pointing substantially in the same direction;
• generating a magnetic flux path in a downhole formation using the core and the winding;
• receiving voltage signals induced by eddy currents flowing in the downhole formation surrounding the open hole associated with the magnetic flux path in a first receiver and a second receiver, the first receiver positioned a first distance from the first core end uphole of the first end and the second receiver spaced a second distance from the second end downhole of the second end;
• and
• differentially amplifying a first signal from the first receiver and a second signal from the second receiver to estimate a parameter of the formation.

A downhole tool for use in an open hole, comprising:

• a winding;
• and
• a core upon which the winding is wound, a first core end and a second core end, the first and second core ends defining an air space therebetween, the winding and the core arranged to induce electromagnetic flux fields in a formation surrounding the open hole so that magnetic flux leaks from the first core end and returns to the second core end with a magnetic flux path through the downhole formation;
• and
• a receiver placed substantially in the air space between the first core end and the second core end and configured to receive signals induced by the core and the winding in the formation surrounding the open hole.

A method for estimating a property of a formation, the method comprising:

• providing a winding;
• providing a core upon which the winding is wound, the core having a first core end and a second core end pointing in substantially the same direction, the first and second core ends defining an air space therebetween;
• generating a magnetic flux in the formation by the core and the winding;
• receiving signals from the formation responsive to the generated electromagnetic flux in a receiver placed substantially in the air space between the first core end and the second core end;
• processing the received signals to estimate the property of the formation;
• and
• recording the estimated property of the formation in a suitable medium.

A system for using a downhole tool in an open hole, the system comprising:

• a surface controller configured to control the downhole tool in the open hole;
• a winding;
• and
• a core upon which the winding is wound, the core having a first core end and a second core end, each core end pointing in a selected direction, the first and second core ends further defining an air space therebetween, the winding and the core arranged to induce electromagnetic flux fields in a formation surrounding the open hole along the selected direction;
• an extendable pad from the tool that encloses the core;
• a receiver spaced apart from the core and placed in a space between the first core end and the second core end, the receiver configured to receive signals corresponding to the induced electromagnetic flux in the formation;
• and
• a processor configure to process the received signals to estimate a property of the formation surrounding the open hole.