hi sunpilot---there is a big difference between the sd--and the 18000---the 18000 is a vlf machine--vlf means "very low frequency" any frequencey below 250 i believe is called a low frequency. the lower the frequency the deeper the machine will search. the higher the frequency the smaller the gold that can be found, but you will loose depth. the minelab xt18000 has a three position switch that allows you to hunt in one of three frequencies--6.4 kh for deeper targets, 20 kh, for general searching, and 60kh for tiny nuggets. it has automatic ground balance, and continual ground tracking, so that it is constantly working at maximum efficiency with little effort on your part. it is absolutely nessesary that your machine be ground balanced. if your machine becomes negatively ground balanced you will loose depth, and miss targets. people with manual ground balanced machines should check there ground balance every 8 feet or so and reground balance if nessesary. the sd machine is a whole different animal.--it is a pulse machine, and uses a mono loop coil for greater depth, and a dd coil if you are using iron identification. (that is a whole nother discussion). in actuallity the sd is two machines in one. minelab calls it a mps, or multi pulse sensing, the machine fires pulses ,alternating from the #1 machine, to the #2 machine. sort of in a rapid fire. thereby giving you more power into the ground vs. the vlf machine---the sd level of machine is able to locate a target of say 5 inches dia. up to 5 feet deep. it's big hole time.the sd loves ground mineralization, the hotter the ground the deeper it will go, and does'nt hear ground "mineralization"--it will pick up iron rocks, or ground with fine iron ore in it. you should swing the sd VERY slowly and as close to the ground as possible. if you have any further questions email me --don't buy on price alone---hoss