A method comprising: a) deploying a logging instrument into a borehole drilled into a formation, the instrument including a pressure sensor and being coupled to a drill string; b) using the pressure sensor to obtain a first pressure value at a known depth in the borehole; c) using the pressure sensor to obtain at least one further pressure value subsequent to the first pressure value during withdrawing the drill string and the logging instrument from the borehole or advancing the drill string and the logging instrument in the borehole; d) using a processor to receive the first pressure value and the at least one further pressure value obtained by the pressure sensor; e) determining, using the processor, an actual depth of the logging instrument within the formation, using: (i) a change in pressure (Δρ) between the first pressure value and a said further pressure value or values; or (ii) a change in pressure (Δρ) between a said further pressure value and another said further pressure value; or (iii) a combination of (i) and (ii); wherein the change in pressure (Δρ) corresponds to a change in a length of the drill string; characterized in that the drill string comprises a plurality of drill rods of known length, wherein the drill string is withdrawn from the borehole or inserted into the borehole in stages to produce distinct detectable pressure changes, wherein each stage corresponds to removal or insertion of a respective known length drill rod of the drill string; and further characterized in that the logging instrument comprises a sonde, wherein the sonde houses the pressure sensor, wherein a drill bit is disposed at a distal end of the drill string, the drill bit having an aperture through which the sonde may be advanced, wherein the sonde is advanced beyond the drill bit and wherein the one or more pressure values is/are sensed by the sensor(s) beyond the distal end of the drill bit. a) deploying a logging instrument into a borehole drilled into a formation, the instrument including a pressure sensor and being coupled to a drill string; b) using the pressure sensor to obtain a first pressure value at a known depth in the borehole; c) using the pressure sensor to obtain at least one further pressure value subsequent to the first pressure value during withdrawing the drill string and the logging instrument from the borehole or advancing the drill string and the logging instrument in the borehole; d) using a processor to receive the first pressure value and the at least one further pressure value obtained by the pressure sensor; e) determining, using the processor, an actual depth of the logging instrument within the formation, using: (i) a change in pressure (Δρ) between the first pressure value and a said further pressure value or values; or (ii) a change in pressure (Δρ) between a said further pressure value and another said further pressure value; or (iii) a combination of (i) and (ii); wherein the change in pressure (Δρ) corresponds to a change in a length of the drill string; characterized in that the drill string comprises a plurality of drill rods of known length, wherein the drill string is withdrawn from the borehole or inserted into the borehole in stages to produce distinct detectable pressure changes, wherein each stage corresponds to removal or insertion of a respective known length drill rod of the drill string; and further characterized in that the logging instrument comprises a sonde, wherein the sonde houses the pressure sensor, wherein a drill bit is disposed at a distal end of the drill string, the drill bit having an aperture through which the sonde may be advanced, wherein the sonde is advanced beyond the drill bit and wherein the one or more pressure values is/are sensed by the sensor(s) beyond the distal end of the drill bit. (i) a change in pressure (Δρ) between the first pressure value and a said further pressure value or values; or (ii) a change in pressure (Δρ) between a said further pressure value and another said further pressure value; or (iii) a combination of (i) and (ii);