It is usual in the oilfield industry to perform various measurements during a well-bore drilling operation. Such measurements are known as measurements-while-drilling (MWD) or logging while drilling (LWD). These measurements relates to well-bore characteristics and to the properties of the geological formation run across during the excavation of the hole, or shortly thereafter. The measurements are performed by tools integrated into a bottom-hole assembly, in particular into a drill-string. The measurements-while-drilling tool typically provides pressure, temperature and well-bore trajectory in three-dimensional space. The logging while drilling tool typically provides geological formation parameters (resistivity, porosity, sonic velocity, gamma ray . . . ). These measurements are performed while extending the well-bore. The measurements which are made down-hole can be stored in a memory and later transmitted to the surface (e.g. data transmission through pressure pulses, later retrieval with wire-line, or retrieval when the tool is tripped out of the hole).
It is also usual in the oilfield industry to perform various measurements after the well-bore drilling operation has been carried out. Such measurements are known as wire-line logging. The wire-line logging employs an electrical cable to lower tools into the borehole and to transmit data measured down-hole to surface equipment. The measurements relates to well-bore characteristics (pressure, temperature, drilling fluid . . . ) and to the properties of the geological formation around the well-bore (resistivity, porosity, sonic velocity, gamma ray . . . ) versus depth or time, or both.
While various well-bore characteristics can be measured, there is currently a need of measuring the dielectric properties, namely permittivity and conductivity of the down-hole material, in particular the geological formation or the drilling mud or the mudcake, in the close vicinity of the well-bore wall.