Abstract: This disclosure discusses methods and apparatus for generating vertical seismic profiles of a subsurface formation though deploying an untethered downhole tool into a wellbore, the untethered tool including at least one seismic sensor, a ballast, and a first electromagnet in the untethered downhole tool to attach the untethered downhole tool to a casing of the wellbore at a predetermined depth. The method further includes transmitting a seismic signal into the subsurface formation from a source located above the subsurface formation, receiving the seismic signal with the at least one seismic sensor, 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.

Abstract: A downhole tool system includes a downhole tool that includes a tool body configured to move within a wellbore, a depth detection sub-assembly and configured to generate a signal based on a known depth location of the tool body in the wellbore, an acoustic transmitter sub-assembly including an acoustic pinger configured to generate acoustic pulses, and a measurement and control sub-assembly configured to receive the signal from the depth detection sub-assembly and, based on the signal, activate the acoustic transmitter sub-assembly to initiate the acoustic pulses from the acoustic pinger. The system further includes a control system that includes a fiber optic interrogator communicably coupled to a fiber strand installed in the wellbore and configured to determine a travel time of the tool body along the fiber strand or a particular distributed acoustic sensing (DAS) channel of a plurality of DAS channels based on a detection of at least one disturbance in the fiber strand caused by the acoustic pulses.