will be provided in association with lower end 32 of return tubing 30 or lower end 42 of supply tubing 40 in a particularly preferred embodiment the lf wave generating means will be incorporated into expander means 50 where expander means 50 is in the form of a flow obstruction it may be adapted to generate lf waves mechanically as shock waves caused by the movement of liquid refrigerant 70 past the flow restriction in alternative embodiments an electromagnetic wave transmitter is provided in association with lower end 32 of return tubing 30 or lower end 42 of supply tubing 40 in such embodiments the amplitude and frequency of lf waves can be regulated by control means not shown located at the surface preferably the lf waves are generated in pulsed fashion which is believed to enhance the effectiveness of the wave energy in advancing the freezing front within formation 20 persons of ordinary skill in the art of the invention will appreciate that mechanical or electromagnetic means for generating lp waves can be provided in a variety of forms using known technology accordingly embodiments of the invention involving the use of lf waves are not to be limited to.