spectrum is the most temperature sensitive with the intensity of the spectrum varying with temperature although brillouin scattering and in certain cases rayleigh scattering are also temperature sensitive generally in one embodiment pulses of light at a fixed wavelength are transmitted from the light source in acquisition unit 28 down the optical fiber 26 at every measurement point in the optical fiber 26 light is back scattered and returns to the acquisition unit 28 knowing the speed of light and the moment of arrival of the return signal enables its point of origin along the optical fiber 26 to be determined temperature stimulates the energy levels of molecules of the silica and of other index modifying additives such as germania present in the optical fiber 26 the back scattered light contains upshifted and downshifted wavebands such as the stokes raman and anti stokes raman portions of the back scattered spectrum which can be analyzed to determine the temperature at origin in this way the temperature of each of the responding measurement points in the optical fiber 26 can be calculated by the acquisition unit 28 providing a complete temperature profile along the length of the optical fiber 26 in one embodiment the optical fiber 26 is disposed in a u shape along the wellbore 12 providing greater resolution to the temperature measurement fig2 shows a graph of the geothermal temperature profile 29 of a generic horizontal wellbore this profile shows at 30 a gradual increase in temperature as the depth of the well increases until at 32 a stable temperature is reached along the horizontal section of the wellbore the geothermal temperature profile is the temperature profile existing in the wellbore without external factors such as injection after injection or other external factors end the wellbore will gradually change in.