Magnetic Tape Recorders for Down-hole Data Recovery and Their Use with Electronic Tubing Calipers
Authors: Anderson Seth A. , Kerbow Odis D. 
Published: December 2007

Abstract
A magnetic tape recorder has been developed for the storage and recovery of data of subsurface instruments. This 14-channel recorder is capable of storing data from 14 independent transducers and may be lowered through production tubing on a solid wireline. The recording technique, the response of the recorder and the temperature limitations of the recorder are briefly described in this paper. A magnetic tubing caliper developed for use with this recorder is also presented. Finally, description of the experimental calipering system used to indicate mental loss in tubing on the surface, as well as the principle of operation of this magnetic caliper when used for subsurface operations, is included.

Introduction
Techniques for recovery of data from oil wells are highly developed and widely practiced. The more intricate techniques usually employ a conductor cable to transmit information to the surface in the form of electrical signals from a transducer which has been lowered into the borehole. Special equipment is required to lower instruments attached to a conductor cable into high-pressure wells. At very high pressures this may not be effective or, at very best, is quite costly. Expensive electronic equipment is required at the wellsite to receive, store and prepare the data for interpretation.A down-hole data-recovery system which contains smaller and often less-complicated subsurface equipment and which is easy to run in producing wells is offered by a magnetic tape recorder with self-contained battery power. Data can be recovered and stored without any electrical communication between the transducer and the surface. A self-contained unit can be lowered into high-pressure wells by means of a solid wireline without special techniques and equipment. The down-hole electronics needed for preparing information for recording is comparable to and, in most cases, less complex than that required to recover the same data by means of an electrical conductor line. No additional electronic equipment which must be operated by competent technicians is needed at the wellsite.A magnetic tape recorder for down-hole data recovery has been tested under field conditions simulated in the laboratory. This multi-channel recorder is capable of recording information from 14 independent sources simultaneously, and can be lowered into production tubing with a nominal ID of 2-in. or larger.

Magnetic Tape Recording
All magnetic tape recording is accomplished by subjecting the ferromagnetic material on the tape to a magnetic field. Particles on the tape become magnetized and retain some of the magnetization after the magnetic field has been removed. Magnetic fields are produced by current in the coils of the record head, and the tape is subjected to these fields by passing the tape over the recording head so that it encounters the flux fringing from the head gap.The playback head of a recording system, which utilizes motion between the head and medium to produce the flux changes and reproduces the signal, is identical or similar in construction to the record head. The voltage induced in the coil of the playback head with sinusoidal residual magnetism on the tape is given by the formula
. . . . . . . . (1)
where k = a constant of proportionality including the number of coil turns on the head and amplitude of the residual magnetism on the medium, f = frequency of the sinusoidal wave, and t = time.
Thus, the output signal is proportional to the amplitude and frequency of the recorded signal. Doubling the reproduced frequency by doubling the velocity of the medium will produce the same increased output as doubling the recording frequency, assuming the recording signal is constant in amplitude. Eq. 1 assumed that the gap in the head was short compared to the wave length of the signal.
JPT
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