Downhole sensing for electromagnetic telemetry

1. A method for recovering data from a downhole tool in a wellbore, comprising:

measuring an electromagnetic signal using first and second sensors, wherein at least a portion of the electromagnetic signal is transmitted by the downhole tool positioned in a first wellbore, and wherein the first and second sensors are each positioned at a different location along a length of a second wellbore;

decoding the electromagnetic signal measured by the first and second sensors to recover a property measured by the downhole tool; and

wherein the electromagnetic signal is measured by the first and second sensors when the downhole tool is in a first portion of the first wellbore, and said method further comprises measuring the electromagnetic signal using the second sensor and a third sensor when the downhole tool is in a second portion of the first wellbore that is axially-offset from the first portion of the wellbore.

2. The method of claim 1 , wherein the first and second sensors are both in contact with an electrically-conductive member that is positioned at least partially within the second wellbore.

3. The method of claim 2 , wherein the electrically-conductive member comprises a casing.

4. The method of claim 3 , wherein a resistance between the first and second sensors is less than or equal to about 25 ohms.

5. The method of claim 1 , wherein the first sensor, the second sensor, or both comprise a toroidal coil.

6. The method of claim 1 , further comprising generating an alert when the electromagnetic signal measured by the first and second sensors exceeds a predetermined threshold.

7. The method of claim 6 , further comprising transmitting a signal to the downhole tool in response to the alert, wherein the signal directs the downhole tool to steer toward a pay zone layer in a subterranean formation.

8. The method of claim 1 , further comprising transmitting a signal to the downhole tool in response to recovering the property, wherein the signal causes the downhole tool to perform a drilling action.

9. The method of claim 8 , wherein the drilling action is selected from the group consisting of: varying a trajectory of the downhole tool, varying a weight-on-bit of the downhole tool, varying a flow rate of fluid being pumped into the first wellbore, varying a composition of the fluid being pumped into the first wellbore, and measuring an additional property in a subterranean formation using the downhole tool.

10. A system for recovering data from a downhole tool in a wellbore, comprising:

a first sensor configured to measure an electromagnetic signal transmitted by the downhole tool in a first wellbore, wherein the first sensor is in a second wellbore;

a second sensor configured to measure the electromagnetic signal transmitted by the downhole tool, wherein the first and second sensors are each positioned at a different location along a length of the second wellbore;

a third sensor configured to measure current transmitted by the downhole tool, wherein the third sensor is positioned above a subterranean formation and proximate to a top of the second wellbore; and

a computer system configured to determine a voltage differential between the first and second sensors and decode the voltage differential to recover a property measured by the downhole tool.

11. The system of claim 10 , wherein the first sensor is in contact with a casing in the second wellbore.

12. The system of claim 11 , wherein the second sensor is also in contact with the casing, and wherein a resistance between the first and second sensors is less than or equal to about 25 ohms.

13. The system of claim 10 , wherein the first sensor, the second sensor, or both comprise a toroidal coil.

14. A method for recovering data from a downhole tool in a wellbore, comprising:

transmitting an electromagnetic signal from a downhole tool in a first wellbore to a second wellbore, the second wellbore intersecting a subterranean payzone;

measuring the electromagnetic signal using first and second sensors deployed in the second wellbore, the first sensor being deployed above the payzone and the second sensor being deployed below the payzone;

decoding the electromagnetic signal measured by the first and second sensors to recover a property measured by the downhole tool; and

evaluating amplitudes of the electromagnetic signals measured at the first and second sensors to determine a location of the downhole tool relative to the payzone.

15. The method of claim 14 , wherein the first and second sensors are each in contact with an electrically-conductive casing that is positioned at least partially within the second wellbore.

16. The method of claim 14 , wherein the amplitudes of the electromagnetic signals measured at the first and second sensors are both small when the first wellbore is located in the payzone.

17. The method of claim 14 , wherein the amplitude of the electromagnetic signal measured at one of the first or second sensors increases when the first wellbore exits the payzone.

18. The method of claim 14 , further comprising

altering a trajectory of a downhole tool when the evaluated amplitudes indicate that the downhole tool is above or below the payzone.