Abstract: A method and apparatus for digitally decoding a biphase encoded transmission signal, which combines both clock and data signals with a high degree of noise immunity. The decoding method has a skew tolerance nearly equivalent to using conventional decoding methods with twice the sampling rate. The method utilizes a unique look-ahead feature to resolve otherwise ambiguous signals.

Abstract: Methods and apparatus are described utilizing low frequency tube waves in determining characteristics of formations traversed by a borehole. Data received from three spaced receiving transducers are utilized in formulating quantities representative of complex compressibility of formations as well as loss parameters attributed to formation characteristics. When these two factors are recorded as a function of depth, interpretation is made readily available as to the permeability and rigidity of subsurface earth formations. Data is also provided by way of the present invention to enable interpretation concerning the presence of fractures or cracks in the earth formations. The methods and apparatus may also be utilized to examine the condition of casing in a borehole.

Abstract: In the representative embodiment of the new and improved perforating apparatus described herein, shaped charge cases are provided with matching upper and lower supports with parallel abutment surfaces and alignment means cooperatively arranged to enable a plurality of these charge cases to be stacked together for erecting an intertwined assembly of cases. By appropriately selecting the combined heights of the matching supports, adjacent ones of the charge cases can be closely spaced one above the other and directed outwardly along selected radial perforating axes at uniform angular spacings.