A system for generating variable frequency tube waves includes a high pressure multiplex pump having a number of plungers, with each plunger operatively coupled to a suction valve on a suction side and a discharge valve on a discharge side. The suction valve or the discharge valve of a first one of the plungers includes an opening, such that the modified plunger on a discharge stroke pushes fluid through the opening in the suction or discharge valve. The system includes a tubular fluidly coupling the high pressure multiplex pump to a wellbore, and a pressure sensor that receives tube waves generated by the high pressure multiplex pump and reflected from the wellbore.