A method of determining subsurface temperatures of a surveyed region comprising the steps of: using a transmitter to transmit a pulsed electromagnetic signal into the ground; using a receiver to detect a return signal following interaction of said transmitted signal with features of the subsurface; performing a first scan to obtain a profile image of the subsurface by repeating said transmit and detect steps at a plurality of different points over said subsurface, in each case with said transmitter and receiver being maintained at a set separation; performing a further scan in order to obtain depth information for said profile image of the subsurface, said first scan and/or said further scan identifying a number of different layers divided by reflective boundaries, wherein there is a plurality of received signals, each obtained from a different layer; determining the temperature of the subsurface as a function of depth using said depth information, wherein said determining the temperature of the subsurface comprises determining a respective temperature for each of a plurality of said layers, wherein the temperature for an individual layer is determined from the layer's corresponding received signal; wherein said determining the temperature of the subsurface comprises obtaining a first temperature estimate from a respective dielectric constant of said layer, from a respective power of said signal for said layer and the cross sectional area of the signal, as determined from the layer's corresponding received signal; wherein said cross sectional area of the signal comprises an area defined by a cross section of a beam of said signal at right angles vertically to a z axis, wherein the z-axis is vertical with respect to the surface of the ground. using a transmitter to transmit a pulsed electromagnetic signal into the ground; using a receiver to detect a return signal following interaction of said transmitted signal with features of the subsurface; performing a first scan to obtain a profile image of the subsurface by repeating said transmit and detect steps at a plurality of different points over said subsurface, in each case with said transmitter and receiver being maintained at a set separation; performing a further scan in order to obtain depth information for said profile image of the subsurface, said first scan and/or said further scan identifying a number of different layers divided by reflective boundaries, wherein there is a plurality of received signals, each obtained from a different layer; determining the temperature of the subsurface as a function of depth using said depth information, wherein said determining the temperature of the subsurface comprises determining a respective temperature for each of a plurality of said layers, wherein the temperature for an individual layer is determined from the layer's corresponding received signal; wherein said determining the temperature of the subsurface comprises obtaining a first temperature estimate from a respective dielectric constant of said layer, from a respective power of said signal for said layer and the cross sectional area of the signal, as determined from the layer's corresponding received signal; wherein said cross sectional area of the signal comprises an area defined by a cross section of a beam of said signal at right angles vertically to a z axis, wherein the z-axis is vertical with respect to the surface of the ground.