Source-receiver Arrays Research Articles

Experimental demonstration of an acoustic time reversal mirror in the ocean

An experiment conducted in the Mediterranean Sea in April 1996 demonstrated that a time reversal mirror (or phase conjugate array) can be implemented to spatially and temporally refocus an incident acoustic field back to its origin. The experiment utilized a vertical source–receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) co-located in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.5 km from the SRA. Phase conjugation was implemented by transmitting a 50-ms pulse from the SRT to the SRA, digitizing the received signal and retransmitting the time-reversed signals from all the sources of the SRA. The retransmitted signal was then received at the VRA. An assortment of runs were made to examine the structure of the focal point region and the temporal stability of the process. The phase conjugation process was extremely robust and stable. There was also excellent agreement between experiment and theory.

Travel time tomography simulations for the test ocean constructed for the 124th Meeting of the Acoustical Society of America

Geometric rays were traced for a variety of source–receiver configurations in the test ocean sound-speed field constructed for the 124th Meeting of the Acoustical Society of America. A stochastic inverse incorporating the statistical information distributed with the test ocean was performed using only one-way travel times as data. Realistic errors in travel times and source–receiver positions were allowed for. The travel times were assumed to be contaminated with internal-wave-induced travel time fluctuations. The fluctuation variance increased linearly with range, with a magnitude of 10 ms rms at 1000 km. This internal wave noise can be reduced by a factor of N−1/2 by assuming that the mesoscale sound-speed structure remains static over N internal-wave-induced decorrelation times. Decorrelation times shorter than 10 min have been observed in the North Pacific at 1000-km range, so a daily average can yield a net contamination below 1 ms, neglecting the effects of longer period fluctuations like internal tides. The simulation results are sensitive to the size of the assumed error, and using values below 1 ms can lead to unrealistically good performance. The size of the source–receiver position errors can vary dramatically depending on the navigational techniques used and can have significant effects on the array design problem. These sensitivities will be explored in the process of choosing a source–receiver array adequate to resolve 96% of the sound-speed variance at 700-m depth in the test ocean.

Exploring for Subtle Sandstone Channels By Use of Electrical Geophysics: ABSTRACT

Electrical methods can be used directly as a resistivity tool which not only defines structure but also provides information, such as rock type, fluid content, and porosity, necessary to resolve lithologic and stratigraphic problems. To help meet the challenge of today's petroleum exploration problems, a multi-methodology electrical resistivity system has been developed. This system uses various source-receiver arrays and multiple source and receiver types. Seismic roll-along cables and an eight-channel digital recording system are used to achieve rapid field coverage. Measurements are made in profile every 220 ft (67 m). Five decades of frequency can be covered to produce soundings from the surface to a maximum of 30,000 ft (9,100 m) in depth. A powerful transmitter is used to obtain near-zone (late time) and far-zone (early time) electromagnetic soundings as well as DC soundings. Magnetotelluric measurements are used to reach depths below 20,000 ft (6,100 m) if necessary. The data are displayed as profiles of closely spaced pseudo E-logs and induction logs that are correlatable to existing well logs. Two case histories of exploration for subtle sand traps include one located in the D J basin of Colorado. The other describes the detection of a 30 ft (9 m) sand sequence at a depth of 2,500 ft (760 m) on the eastern shelf of the Permian basin in west Texas. The approaches used in these case histories have direct application to many problems of the central valleys of California. End_of_Article - Last_Page 1691------------