Apparatus for superimposing a seismic wave onto a flow of liquid being injected into the ground around a borehole, characterised by the following combination of features: the apparatus includes a reservoir for storing, under pressure, the liquid to be injected into the ground; the apparatus includes a downhole-structure, which is suitable for lowering down into the borehole; the downhole-structure has an overall-circumference, being the circumference that circumscribes all portions of the down-hole structure, the overall-circumference measures OC cm in length, measured in a plane at right angles to the axis of the borehole; the downhole-structure includes a valve, comprising a valve-face (35, 110, 123, 154) on a valve-member, and a valve-seat (34, 109, 125, 158) which is movable relative to the valve-member; the valve is operable between a closed condition and a full-open condition; in the full-open condition, the valve-face (35, 110, 123, 154) lies separated from the valve-seat (34, 109, 125, 158) and lies spaced therefrom by a valve-gap; in the closed condition, the valve-face (35, 110, 123, 154) and the valve-seat (34, 109, 125, 158) make sealing contact over an annular contact-area; a valve-gap-seal-contact-line is defined as the locus of the mid-point of the said annular contact-area at each radius around the valve-gap-seal-contact-line; the valve-gap-seal-contact-line has a valve-gap-seal-circumference (VGSC) that measures VGSC cm; and VGSC (valve-gap-seal-circumference) is larger than seventy-five percent of OC (overall circumference) and in that the downhole-structure includes a fixed housing (46, 127, 160), and the valve-member comprises a slider (24, 75, 84, 93, 120, 156) that can slide axially within the housing (46, 127, 160), between the open and closed conditions, and the apparatus includes an operable valve-activation motor which is effective, when operated, to so move the slider (24, 75, 84, 93, 120, 156). the apparatus includes a reservoir for storing, under pressure, the liquid to be injected into the ground; the apparatus includes a downhole-structure, which is suitable for lowering down into the borehole; the downhole-structure has an overall-circumference, being the circumference that circumscribes all portions of the down-hole structure, the overall-circumference measures OC cm in length, measured in a plane at right angles to the axis of the borehole; the downhole-structure includes a valve, comprising a valve-face (35, 110, 123, 154) on a valve-member, and a valve-seat (34, 109, 125, 158) which is movable relative to the valve-member; the valve is operable between a closed condition and a full-open condition; in the full-open condition, the valve-face (35, 110, 123, 154) lies separated from the valve-seat (34, 109, 125, 158) and lies spaced therefrom by a valve-gap; in the closed condition, the valve-face (35, 110, 123, 154) and the valve-seat (34, 109, 125, 158) make sealing contact over an annular contact-area; a valve-gap-seal-contact-line is defined as the locus of the mid-point of the said annular contact-area at each radius around the valve-gap-seal-contact-line; the valve-gap-seal-contact-line has a valve-gap-seal-circumference (VGSC) that measures VGSC cm; and VGSC (valve-gap-seal-circumference) is larger than seventy-five percent of OC (overall circumference) and in that the downhole-structure includes a fixed housing (46, 127, 160), and the valve-member comprises a slider (24, 75, 84, 93, 120, 156) that can slide axially within the housing (46, 127, 160), between the open and closed conditions, and the apparatus includes an operable valve-activation motor which is effective, when operated, to so move the slider (24, 75, 84, 93, 120, 156).