Downhole logging tool

1. An untethered downhole tool, comprising:

a first body, comprising:

a first housing that defines a first inner volume, and

a first magnetic anchor configured, in an active state, to attach the first body to a wellbore tubular;

a second body, comprising:

a second housing that defines a second inner volume, and

a second magnetic anchor configured, in an active state, to attach the second body to the wellbore tubular;

at least one logging sensor positioned in or on at least one of the first or second housings;

a motor attached to one of the first or second bodies and coupled to a spooler, the motor configured, when activated, to operate the spooler to spool or unspool a cable coupled to the spooler and attached to the other of the first or second bodies; and

at least one controller positioned in at least one of the first or second housings and communicably coupled to the motor and the first and second magnetic anchors, the controller configured to perform operations comprising:

adjusting one of the first or second magnetic anchors from the active state to an inactive state to detach the respective first or second body from the wellbore tubular; and

activating the motor to operate the spooler to spool the cable from the spooler to increase a length of a portion of the cable between the spooler and the detached first or second body.

2. The untethered downhole tool of claim 1 , wherein the controller is configured to perform operations further comprising:

determining that the length of the portion of the cable is at a predetermined length; and

based on the determination, deactivating the motor and adjusting the one of the first or second magnetic anchors from the inactive state to the active state to re-attach the respective first or second body to the wellbore tubular such that the first and second bodies are both attached to the wellbore tubular a distance substantially equal to the predetermined length.

3. The untethered downhole tool of claim 2 , wherein the operation of determining that the length of the portion of the cable is at the predetermined length comprises:

determining a number of revolutions completed by the spooler to spool the cable from the spooler to increase the length of the portion of the cable between the spooler and the detached first or second body;

calculating the length of the portion of the cable based on the determined number of revolutions; and

comparing the calculated length to the predetermined length.

4. The untethered downhole tool of claim 2 , wherein the controller is configured to perform operations further comprising:

adjusting the other of the first or second magnetic anchors from the active state to the inactive state to detach the other of the first or second body from the wellbore tubular;

activating the motor to operate the spooler to spool the cable on the spooler to decrease the length of the portion of the cable between the spooler and the detached first or second body;

determining that the length of the portion of the cable is at a minimum length; and

based on the determination, deactivating the motor and adjusting the other of the first or second magnetic anchors from the inactive state to the active state to re-attach the other of the first or second body to the wellbore tubular such that the first and second bodies are both attached to the wellbore tubular a distance substantially equal to the minimum length.

5. The untethered downhole tool of claim 1 , wherein the controller is configured to perform operations further comprising:

identifying logging measurements from the at least one sensor; and

storing the identified logging measurements in at least one memory of the controller.

6. The untethered downhole tool of claim 1 , wherein the at least one controller comprises:

a first controller positioned in the first housing; and

a second controller positioned in the second housing.

7. The untethered downhole tool of claim 6 , wherein the first controller is communicably coupled to the second controller through the cable.

8. The untethered downhole tool of claim 7 , wherein the second controller comprises a master controller, and the first controller comprises a slave controller.

9. The untethered downhole tool of claim 1 , wherein the first body has a first weight, and the second body has a second weight greater than the first weight.

10. The untethered downhole tool of claim 9 , wherein the first housing is formed of a first material having a first density, and the second housing is formed from a second material having a second density greater than the first density.

11. The untethered downhole tool of claim 1 , wherein at least one of the first or second inner volumes is configured to enclose a pressurized fluid.

12. The untethered downhole tool of claim 1 , wherein at least one of the first or second magnetic anchors comprises two permanent magnets connected in parallel between two ferromagnetic poles, with a first permanent magnet of the two permanent magnets having a higher coercivity or resistance than a second permanent magnet of the two permanent magnets.

13. The untethered downhole tool of claim 12 , further comprising a coating applied to at least a portion of an exterior surface of each of the two ferromagnetic poles, the coating comprising a high friction material relative to the exterior surface.

14. A method, comprising:

inserting an untethered downhole tool into a wellbore that comprises a wellbore tubular at or near a terranean surface, the untethered downhole tool comprising:

a first body comprising a first housing that defines a first inner volume, and a first magnetic anchor coupled to the first body;

a second body comprising a second housing that defines a second inner volume, and a second magnetic anchor coupled to the second body to the wellbore tubular;

at least one logging sensor positioned in or on at least one of the first or second housings;

a motor attached to one of the first or second bodies and coupled to a spooler that includes a cable coupled to the spooler and attached to the other of the first or second bodies; and

at least one controller positioned in at least one of the first or second housings and communicably coupled to the motor and the first and second magnetic anchors;

adjusting, with the controller, one of the first or second magnetic anchors from an active state in which the one of the first or second magnetic anchors magnetically attaches the respective first or second body to the wellbore tubular to an inactive state to detach the respective first or second body from the wellbore tubular; and

activating, with the controller, the motor to operate the spooler to spool the cable from the spooler to increase a length of a portion of the cable between the spooler and the detached first or second body.

15. The method of claim 14 , further comprising:

determining, with the controller, that the length of the portion of the cable is at a predetermined length; and

based on the determination, deactivating, with the controller, the motor and adjusting the one of the first or second magnetic anchors from the inactive state to the active state to re-attach the respective first or second body to the wellbore tubular such that the first and second bodies are both attached to the wellbore tubular a distance substantially equal to the predetermined length.

16. The method of claim 15 , wherein determining that the length of the portion of the cable is at the predetermined length comprises:

determining, with the controller, a number of revolutions completed by the spooler to spool the cable from the spooler to increase the length of the portion of the cable between the spooler and the detached first or second body;

calculating, with the controller, the length of the portion of the cable based on the determined number of revolutions; and

comparing the calculated length to the predetermined length.

17. The method of claim 15 , further comprising:

adjusting, with the controller, the other of the first or second magnetic anchors from the active state to the inactive state to detach the other of the first or second body from the wellbore tubular;

activating, with the controller, the motor to operate the spooler to spool the cable on the spooler to decrease the length of the portion of the cable between the spooler and the detached first or second body;

determining, with the controller, that the length of the portion of the cable is at a minimum length; and

based on the determination, deactivating, with the controller, the motor and adjusting the other of the first or second magnetic anchors from the inactive state to the active state to re-attach the other of the first or second body to the wellbore tubular such that the first and second bodies are both attached to the wellbore tubular a distance substantially equal to the minimum length.

18. The method of claim 14 , further comprising:

identifying, with the controller, logging measurements from the at least one sensor; and

storing, with the controller, the identified logging measurements in at least one memory of the controller.

19. The method of claim 14 , wherein the at least one controller comprises:

a first controller positioned in the first housing; and

a second controller positioned in the second housing.

20. The method of claim 19 , further comprising providing at least one of data or power from the first controller to the second controller through the cable.

21. The method of claim 20 , wherein the second controller comprises a master controller, and the first controller comprises a slave controller.

22. The method of claim 14 , wherein the first body has a first weight, and the second body has a second weight greater than the first weight.

23. The method of claim 22 , further comprising forming the first housing of a first material having a first density, and forming the second housing from a second material having a second density greater than the first density.

24. The method of claim 14 , further comprising filling at least one of the first or second inner volumes with a pressurized fluid.

25. The method of claim 15 , wherein at least one of the first or second magnetic anchors comprises two permanent magnets connected in parallel between two ferromagnetic poles, with a first permanent magnet of the two permanent magnets having a higher coercivity or resistance than a second permanent magnet of the two permanent magnets, and

adjusting the one of the first or second magnetic anchors from the inactive state to the active state comprises:

controlling a power source of the respective first or second bodies to provide power to a coil of the second permanent magnet;

generating an external flux in a first direction through the second permanent magnet; and

extending at least a portion of one or more magnetic field lines from the two ferromagnetic poles to attach to the wellbore tubular.

26. The method of claim 25 , further comprising adjusting the one of the first or second magnetic anchors from the active state to the inactive state by:

controlling the power source of the respective first or second bodies to provide power to the coil of the second permanent magnet;

generating the external flux in a second direction through the second permanent magnet; and

retaining a portion of the one or more magnetic field lines within the two ferromagnetic poles to detach from the wellbore tubular.