GB777647A - Method and apparatus for cancelling reverberations in water layers - Google Patents

Abstract

777,647. Geophysical prospecting. TEXAS INSTRUMENTS, Inc. May 18, 1955, No. 14376/55. Drawings to Specification. Class 118 (2). A method of making a seismic survey record of a water-covered area wherein the bottom is a good reflector causing standing waves to be set up by reverberations in the water layer comprises using two seismometers positioned to receive the said standing waves 180 degrees outof phase and combining the outputs of the seismometers such that these unwanted signals cancel out. It is pointed out that location of seismometers at standing wave nodes would have to be precise whereas positioning them at antinodes is not very critical. Several arrangements of seismometer spreads are described wherein two seismometer streamers are moved through the water at two distinct depths. The depths chosen will vary according to whether pressure or velocity sensitive types of seismometer are used, and to which harmonic of the fundamental frequency is used. Use of an amplifier with a suitable bandpass characteristic avoids unwanted harmonics (though perhaps received at one detector) from being recorded or interfering with the cancellation effect. In Fig. 5 (not shown) a ship tows a floating cylindrical " fish " carrying two reversible motors controlled from the ship to lower two weights respectively. Each weight has attached to it a seismometer streamer and may carry pressuresensing elements connected to instruments on board the ship to help in maintaining the streamers at the antinode depths calculated from knowledge of the depth of the water layer. If velocity type seismometers are used for the top streamer this streamer may be positioned at the velocity antinode of the fundamental frequency of the standing wave, viz., the water surface. Apparatus wherein the depths of the spreads are fixed before being placed in the water and remain constant throughout the recording process may consist of a steel tension member and a conductor cable bound together and supported in the water by a series of floats, seismometers being suspended from this at the required depths for each particular seismic prospect. The seismometers of the lower spread may be suspended from those of the upper spread as in Fig. 6 (not shown). Since the velocity antinode of the fundamental frequency exists at the water surface whilst the pressure antinode exists at the bottom, using appropriate seismometers one spread may be positioned near the surface by a floating cable and the other spread near the bottom by floating the seismometers on short leads from a heavier-than-water cable, as in Fig. 7 (not shown).