Autonomous untethered well object

1. A method comprising:

deploying an untethered object through a passageway of a string in a well;

acquiring a plurality of measurements representing an environment of the string as the object is being communicated through the passageway;

cross-correlating the plurality of measurements; and

using results of the cross-correlating to identify at least one downhole feature,

wherein using the results of the cross-correlating comprises:

identifying a time shift between at least two measurements of the plurality of measurements; and

using the identified time shift to determine a speed of the untethered object, and

wherein using the result of the cross-correlating to identify the time shift comprises identifying the time shift based at least in part on a maximum of the cross-correlating evaluated at different candidate time shifts.

2. The method of claim 1 , wherein using the result of the cross-correlating further comprises:

using at least two measurements of the plurality of measurements to identify times at which the object passes in proximity to at least two features of the string; and

identifying a location of the object based at least in part on the speed and the times.

3. The method of claim 1 , wherein acquiring the plurality of measurements comprises:

driving a first coil of the object to produce a magnetic field that is influenced by the string;

acquiring signals representing at least two of the measurements from second and third coils in response to the magnetic field.

4. The method of claim 3 , wherein driving the first coil comprises driving a coil of a balanced coil sensor.

5. The method of claim 1 , further comprising:

selectively autonomously operating the untethered object in response to identifying the at least one downhole feature.

6. The method of claim 5 , wherein selectively autonomously operating the untethered object comprises selectively radially expanding the untethered object.

7. An apparatus usable with a well, comprising:

a string comprising a passageway; and

an untethered object adapted to be deployed in the passageway, such that the untethered object travels in the passageway, the object comprising:

a magnetic field generator;

antennae spatially separated to provide a plurality of signals generated in response to a magnetic field generated by the magnetic field generator;

an expandable element; and

a controller to:

cross-correlate the signals;

use the cross-correlation of the signals to identify at least one downhole feature of the string; and

selectively radially expand the element based at least in part on the at least one identified downhole feature,

wherein the controller is adapted to:

determine a speed of the object based at least in part on the result of a cross-correlation;

cross-correlate the signals for different candidate time shifts;

identify a maximum correlation for the candidate time shifts; and

base the determination of the speed based at least in part on the identified maximum correlation.

8. The apparatus of claim 7 , wherein the untethered object comprises a balanced coil sensor comprising the antennae.

9. The apparatus of claim 7 , wherein the controller is adapted to identify the at least one downhole feature based at least in part on a determined speed of the object and known distances between features of the string.

10. A method comprising:

deploying an untethered object through a passageway of a tubular member;

acquiring a plurality of measurements representing an environment of the tubular member as the object is being communicated through the passageway;

cross-correlating the plurality of measurements; and

using results of the cross-correlating to identify at least one feature of the tubular member,

wherein using the results of the cross-correlating comprises:

identifying a time shift between at least two measurements of the plurality of measurements; and

using the identified time shift to determine a speed of the untethered object, and

wherein using the result of the cross-correlating to identify the time shift comprises identifying the time shift based at least in part on a maximum of the cross-correlating evaluated at different candidate time shifts.