A new type of distributed charge intended to be a seismic source in geophysical prospecting is described; it uses a number of lump charges, located in a shothole at uniformly spaced separations, connected by a time‐delay fuse. The time‐delay fuse consists of appropriate lengths of linear detonating explosive (Primaline®) in contact with a nondetonating linear deflagrator (Nonel®). Nonel is a nonelectrical delay detonator invented by Nitro Nobel AB of Sweden and licensed to Ensign Bickford for manufacturing and marketing Nonel‐based products. Field tests performed near Tulsa, Oklahoma, determined that (1) Nonel deflagration was both consistent and reliable for pressures from atmospheric to 150 psi, (2) Nonel did not cross‐detonate even when “bundled,” and (3) distributed charges using Nonel as an element in the timing fuse detonated 26 days after being loaded in a shothole. Field tests of distributed charges using Nonel performed in central Louisiana showed that (1) distributed charges increase the signal‐to‐noise (S/N) ratio relative to that yielded by concentrated charges by reducing shot‐generated surface noise, (2) distributed charges increase the usable frequency content over that yielded by concentrated charges, and (3) a reliable and efficient distributed charge can be constructed with Nonel Primadet delay cord.

The effect of hydrophone arrays in the recording of seismic signals during offshore Texas seismic marine experiments is judged by comparing traces of spatially tapered hydrophone array signals with traces that are combinations of simultaneously recorded wavetest hydrophone signals. Each spatially tapered hydrophone group array consists of 26 hydrophones nonuniformly spaced over 212 ft. The wavetest streamer section consists of 36 groups of two hydrophones, each pair connected in parallel and with hydrophones back‐to‐back for acceleration cancellation, with 5-ft spacing between groups. Reflection from deep subsurface interfaces are negligibly affected by hydrophone arrays except for very long arrays and/or long‐range distances. Consequently, the report is primarily concerned with the effects of simulated and real hydrophone arrays on first‐arrival signal and early subbottom reflections. Comparison of theoretical and actual seismic traces from an Aquapulse source for near range distances (835 ft) used in normal operations indicates that (1) near‐simultaneous arrival of the direct wave and surface reflection result in their virtual cancellation, (2) the early event with largest amplitude is associated with constructive interference between source and receiver ghost reflections, and (3) the “pseudo‐bubble” period effectively fixed the predominant frequency of all seismic events at values near 28 Hz. At medium range distances (4755 ft), such comparisons indicate that (1) first arrivals are refracted waves traveling in subbottom layers; (2) the water‐bottom reflection is beyond critical angle and is, therefore, complex; (3) the early events with largest amplitude are multiple reflections; and (4) at least two orders of water‐bottom multiples are identified. The attenuation of the high‐amplitude, first‐arrival signal that includes the water‐bottom reflection permits greater dynamic range in field recording and higher levels of “true” amplitude for later reflections without overload distortion of early events on playback. However, if improved resolution of reflection from moderate depths (∼4000 ft) is important, then arrays of length studied in this report (∼200 ft) should not be used to record signals at range distances greater than about 2000 ft because frequencies above 50 Hz are attenuated severely. Spectral analysis of wavetest records in the absence of signals shows that the wavenumber distribution of the noise is located along a slope line equivalent to 5000 ft/sec between wavenumbers that imply a spectral distribution of 30 to 100 Hz. Theoretical array response studies show that both the 36‐element Chebyshev array and the 26‐element spatially tapered array are superior to a 36‐element uniformly weighted array in rejection of seismic noise in the spectral range of 30 to 100 Hz.

Pressure amplitudes were determined for various kinds of seismic signals observed on special test records obtained during field tests conducted along a 14,000-ft seismic lines in Eugene Island Block 184, offshore Louisiana. Vibrators attached to a Seismograph Service Corp. (SSC) boat generated swept‐frequency and monofrequency signals. Signals from detectors on a streamer cable towed by the boat were recorded by an SSC recording system. Signals from a vertical spread of detectors were recorded by a DFS/9000 recorder on the Transco 184 platform centrally located in the test area. Location of the boat was determined by analysis of time relations of signals from responders located at established positions some distance from the test area. Clock times from manually referenced timing code generators were recorded by both the SSC and DFS recorders to permit synchronization between separately recorded signals. The signals analyzed were separated into three classes: [Formula: see text] includes direct and refracted waves; [Formula: see text] consists of primary reflections; and [Formula: see text] includes signals diffracted from scatterers. The average level of first‐arrival signal [Formula: see text] and reflected signal [Formula: see text] for frequency sets 25, 40, 42.2, 50, and 70.4 Hz in the range of 1414 and 2143 ft, which encompasses streamer cable single‐detector groups, is 337 and 29.6 microbars, respectively. The amplitude of signals [Formula: see text], believed to be diffracted from the contact between key reflectors and a salt dome, ranges from 13 to 20 microbars and is 10 to 100 times the amplitudes of towing and ambient noise, respectively. The observed decay of first‐arrival signal amplitude is approximately proportional to the square root of range distance, or about 2 dB/1000 ft. The observed decay of reflected signal amplitude with range distance is approximately 1 dB/1000 ft.

Seismic field tests conducted near Tulsa, Oklahoma, and the Handy area in north‐central Texas used linear arrays of vibrators to concentrate sound waves into a beam, which is directed vertically when all vibrators operate in‐phase or at an angle from the vertical when relative time delays are introduced to each vibrator. The sound wave directivity was verified in the Tulsa area by recordings from subsurface seismometers, and at the same time reflection enhancement by wave beaming was exhibited from surface seismometers. Visual inspection and statistical analysis of reflection continuity indicated that it makes no difference in the Handy area whether beam‐forming techniques are used in the field or are applied later in processing. This result was anticipated, since previous seismic work in the Handy area indicated that the random noise level was low enough to minimize the theoretical advantage of field summing.