Referring to an induction log made with a particular array of transmitter and receiver coils. The array was introduced in 1960 and became the industry standard for 30 years. The 6FF40 array has six coils with the main transmitter-receiver pair spaced 40 in. (102 cm) apart. The design of the array includes the spacing between the coils, the number of turns and the polarity of each coil. The three transmitter and the three receiver coils are each connected in series to produce one signal output.

FF means focused both radially and vertically. Unlike in an array tool, the focusing is fixed by the hardware design. The 6FF40 was designed to read deep into the formation while minimizing the signal close to the tool and maintaining reasonable vertical resolution. The 6FF40 was combined with a shallow electrode device to form the induction electrical survey. The use of both tools gave a qualitative indication of invasion.

Both the deep induction and the 6FF40 had deconvolution and a skin effect correction applied. The deconvolution was designed to reduce the effect of shoulder beds on the readings in high-resistivity beds. It was not effective in high-contrast formations. The skin effect correction was a simple exponential fit that would work on an analog computer.