The special depth checking apparatus is constructed by cable head eqipped on top and accentuating part eqipped on bottom of micro-magnetic localizer.

The depth checking method is combined with special depth checking apparatus, may achieve all depth checking datas by once drop of apparatus into oil well.

So it indicates that the surface with this kind of concave can reduce friction under the condition of plane-plane contact, and if the concave on the surface has some special depth t...