A downhole cement plug includes a casing collar locator (CCL) operable to generate signals indicative of casing collars on a downhole casing string of a wellbore system; a controller communicably coupled to the CCL to output a plurality of distinct frequency signals based on the signals from the casing collar locator; and a signal generator communicably coupled to the controller to receive the plurality of distinct frequency signals from the controller and transmit the plurality of distinct frequency signals to a terranean surface through a portion of a wellbore system.
