Untethered downhole tools and related methods of generating vertical seismic profiles

A method for generating vertical seismic profile of a subsurface formation, the method comprising:

• deploying an untethered downhole tool into a wellbore, the untethered tool comprising:
• at least one seismic sensor;
• and
• ballast such that, with the ballast, the untethered downhole tool has a higher density than fluid in the wellbore and, without the ballast, the untethered downhole tool has a lower density than fluid in the wellbore;
• activating a first electromagnet in the untethered downhole tool to attach the untethered downhole tool to a casing of the wellbore at a predetermined depth;
• transmitting a seismic signal into the subsurface formation from a source located above the subsurface formation;
• receiving, with the at least one seismic sensor, the seismic signal;
• deactivating the first electromagnet to release the untethered downhole tool from the casing of the wellbore;
• releasing the ballast from the untethered downhole tool such that buoyancy effects move the tool uphole;
• and
• retrieving the untethered downhole tool from the wellbore.