The four different strengths:
*"Traces" - Shows for ore that is 1 tile away. Unlocked with Analyse at prospecting level 20.
*"Slight traces" - Shows for ore that is 2 tiles away. Unlocked with Analyse at prospecting level 20. (Needs confirmation)
*"Faint traces" - Shows for ore that is 3 tiles away. Unlocked at prospecting level 35.
*"Minuscule traces" - Shows for ore that is 4 tiles away. Unlocked at prospecting level 40. (Needs confirmation)