Abstract: An acoustic well logging method employing a tool having at least two acoustic transmitters and at least two acoustic receivers. In a preferred embodiment, one receiver is positioned midway between the two transmitters, and the other receiver is spaced from the first receiver along the longitudinal axis of the tool between the transmitters. Two signals, each representing an acoustic wave that has propagated from a transmitter through a portion of the formation surrounding the borehole to a receiver, are recorded when the tool is in each of two positions relative to the longitudinal axis of the well. In each of the tool positions, one recorded signal is associated with the midpoint receiver and the other with another receiver. From the four recorded signals, a new signal is generated. This derived signal has Fourier amplitude spectrum indicative of the formation parameters, but is independent of components associated with the transmitters and receivers.

Abstract: 2. For use with a cryptographic teletype transmission system having a relay for transferring teletype characters to an encyphering transmitter, a decyphering receiver and a teletype printer, a security system comprising a generator for producing an auxiliary teletype character, circuit means responsive to the failure of transfer of a valid teletype character to said transmitter to activate said generator to supply an auxiliary character to said transmitter, a clamp circuit responsive to activation of said generator to inhibit transfer of a valid character to said transmitter until said auxiliary character has been completed, a transfer system for transferring the decyphered characters from the receiver to the printer, storage means for storing a plurality of successive teletype characters, a recognition circuit responsive to the presence of said auxiliary teletype character in said storage means for inhibiting printing of said auxiliary teletype character.

Abstract: A seismic signal is generated having a pattern which is periodically repetitive. The signal within each period has a nonrepetitive pattern and each period has a time duration not less than the time required for the seismic wave to travel to the deepest subsurface strata of interest and back to the detecting location.

Abstract: A monochromatic interference signal is removed from seismic data records by transmitting into the earth a seismic signal having a null component in its power spectrum at substantially the frequency of the interference signal. When cross-correlation techniques are used to determine subsurface characteristics, the cross-correlation product of the detected signal with the transmitted signal will contain a substantially zero component at the frequency of the interference signal.