Electromagnetic sensor for a downhole dielectric tool
Electromagnetic sensors which provide high coupling gain between the transmitter and receiver of a dielectric tool include a conductive pad having a cavity filled with dielectric filler, and an electrical conductor that provides electrical current to a conductive feeder placed inside the dielectric filler. During operation, current is fed to the conductive feeder to thereby produce an electric monopole that feeds electromagnetic power into the cavity, which then radiates out into the formation as electromagnetic signals. Because of the design of the electromagnetic sensors, the transmitter/receiver coupling gain and SNR are optimized over the entire range of the dielectric constant (i.e., 5≤ϵ r ≤80).
1. An electromagnetic sensor for a downhole logging tool, comprising:
a conductive pad having a cavity therein;
dielectric filler positioned within the cavity;
an electrical conductor coupled to the dielectric filler; and
a conductive feeder positioned inside the dielectric filler, the conductive feeder being coupled to the electrical conductor and further comprising:
a frustum having a first end and a second end opposite the first end, the first end being coupled to a conductor; and
a loading cylinder coupled to the second end of the frustum.
2. An electromagnetic sensor as defined in claim 1 , wherein:
the electrical conductor is a coaxial cable comprising an outer conductor and an inner conductor;
the outer conductor extends up to an edge of the cavity; and
the inner conductor extends into the cavity and dielectric filler, whereby the inner conductor is coupled to the conductive feeder.
3. An electromagnetic sensor as defined in claim 2 , wherein a dimension of the cavity is configured to resonate at an operating frequency of the logging tool.
4. An electromagnetic sensor as defined in claim 2 , wherein dimensions of the cavity and dielectric filler are configured such that an operating frequency of the logging tool is higher than a cut-off frequency of an eigenmode of the logging tool.
5. An electromagnetic sensor as defined in claim 2 , wherein:
the electromagnetic sensor forms part of a downhole logging tool having at least two of the electromagnetic sensors; and
dimensions of the inner conductor, frustum and loading cylinder are configured to maximize coupling between the electromagnetic sensors.
6. An electromagnetic sensor as defined in claim 2 , wherein:
an operating frequency of the downhole tool is approximately 1 Gigahertz;
the inner conductor comprises:
a radius of 0.255 millimeter (“mm”); and
a length of 1 mm;
the frustum comprises a length of 2.5 mm; and
the loading cylinder comprises:
a radius of 1.5 mm; and
a length of 1.5 mm.
7. An electromagnetic sensor as defined in claim 1 , wherein the dielectric filler has a dielectric constant satisfying a criteria defined by:
f
operating
>
1
2
π
μϵ
0
ϵ
r
(
m
π
a
)
2
+
(
n
π
b
)
2
8. An electromagnetic sensor as defined in claim 1 , wherein the conductive feeder is an electric monopole.
9. An electromagnetic sensor as defined in claim 1 , wherein the conductive feeder does not contact the conductive pad.
10. An electromagnetic sensor as defined in claim 1 , wherein the conductive feeder and the conductive pad are metal.
11. An electromagnetic sensor as defined in claim 1 , wherein the electromagnetic sensor forms part of a high frequency dielectric logging tool having two or more of the electromagnetic sensors.
12. An electromagnetic sensor as defined in claim 10 , wherein the logging tool is positioned along a wireline or drilling assembly.
13. An electromagnetic sensor for a downhole logging tool, comprising:
a conductive pad having a cavity therein;
dielectric filler positioned within the cavity; and
an electric monopole feeder positioned inside the dielectric filler, the electric monopole feeder comprising:
a frustum having a first end and a second end opposite the first end, the first end being coupled to a current source; and
a loading cylinder coupled to the second end of the frustum.
14. An electromagnetic sensor as defined in claim 13 , wherein:
the current source is a coaxial cable comprising an outer conductor and an inner conductor;
the outer conductor extends up to an edge of the cavity; and
the inner conductor extends into the cavity and dielectric filler, whereby the inner conductor is coupled to the electric dipole feeder.
15. An electromagnetic sensor as defined in claim 13 , wherein the electromagnetic sensor forms part of a logging tool having two or more of the electromagnetic sensors.
16. An electromagnetic sensor as defined in claim 15 , wherein the logging tool is positioned along a wireline or drilling assembly.
17. A logging method, comprising:
deploying a logging tool into a borehole extending along a formation, the logging tool having two or more electromagnetic sensors comprising:
a conductive pad having a cavity therein;
dielectric filler positioned within the cavity;
an electrical conductor coupled to the dielectric filler; and
a conductive feeder positioned inside the dielectric filler, the conductive feeder being coupled to the electrical conductor and further comprising:
a frustum having a first end and a second end opposite the first end, the first end being coupled to a conductor; and
a loading cylinder coupled to the second end of the frustum;
emitting an electromagnetic signal into the formation;
receiving the signal using at least one of the electromagnetic sensors; and
determining at least one characteristic of the formation using the signal.
18. A logging method as defined in claim 17 , wherein the electromagnetic sensors operate in contact with a wall of the borehole.
19. A logging method as defined in claim 17 , wherein the logging tool is deployed along a wireline or drilling assembly.