1. Installation for measuring the thickness of a ferromagnetic pipe using the detection of the far field of eddy currents, comprising: a housing connected to a logging cable for input into a borehole pipe having an emitting frame and a plurality of symmetrically arranged receiving devices on opposite sides of the emitting frame; circuit for exciting the emitting frames at a selectable frequency; a circuit for receiving a signal from each receiving device and for processing a specified signal with the exception of a double indication of defects. 2. Installation according to claim 1, in which each receiving device is a frame centered on the longitudinal axis of the installation. The apparatus of claim 1, wherein the frequency is selectable between 8.75; 17.5; 35 and 70 kHz. 4. Installation according to claim 1, in which the receiving frame is separated by a distance which, when divided by the inner diameter of the pipe, is greater than or equal to 2.5.5. The installation according to claim 1, in which the set of symmetrically spaced receiving frames consists of four frames, with each pair located on each side of the emitting frame at a distance L1 = k1 × dz and L2 = k2 × dz, where k1 and k2 do not have a common divider and dz is the stride length along the longitudinal axis of the installation. 6. The apparatus of claim 1, wherein the circuit for receiving a signal and for processing said signal with the exception of a double indication of defects is a programmable digital computer equipped with a central processor, a storage device and connections with an analog-to-digital converter to sample the signal into discrete data for processing programmable digital computer. 7. Installation according to claim 1, in which the receiving devices are selected from one�
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