Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
L6
F- 8852
VERTICALLY DIRECTIVE ARRAYS
FOR MARINE SEISMIC EXPLORATION
sackground o~ the Inventlon
This invention relates to methods of and apparatus
for marine seismic exploration, and more particularly to
arrays of sources and receivers which have vertical directivity.
In marine seismic exploration the seismic energy can
be generated by a linear array of sources which are towed behind
the boat. The towing cables provide a fixed spacing between
the sources. Typically, each source may be an "air gun" which
releases pulses of compressed air into the water. A typical
air gun is shown in U.S. patent 3,506,085 to George B. Loper.
Such guns emit seismic pulses having a charactertistic frequency
including the lowest requency, the highest: frequer.cy and the
predominant frequency o~ the seismic energy in each pulse.
The boat also tows a streamer of hydrophones which
detect the seismic energy reflected from subsurface forrnations.
Systems which have a long offset between the sources and the
hydrophones are subject to high amplitude nearly horizontal
traveling source-generated noise.
It can be shown that there is a critical distance
from the source which is determined by the acoustic velocity
in the water and the acoustic velocity in the bottom. Beyond
the critical distance, energy is totally reflected and propa-
gates horizontally. It is desirable to use sources and
receivers which discriminate against such horizontal
propagation.
In the prior art the array length and the spacing
between the elements of the array have been determined by
the predominant frequency in the seismic pulse. Such techniques
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provide directivity for energy at the predominant frequency
but they do not give good directivity over the broad band of
the seismic pulse. For example, in U.S. patent 3,479,638,
the spacing between the sources is an integer of one-half the
wavelength of the desired frequency. In U.S. patent 3,613,823,
the length of the array is greater than one wavelength of the
predominant frequency. Such approaches will not give direc-
tivity over the broad band of the seismic pulse.
;
; Summary of the Invention
In accordance with this invention, vertically
directive arrays of sources and detectors are used to discrim-
inate horizontally propagated source-generated noise in marine
seismic exploration.
Vertical directivity over the broad band of the
seismic pulse is obtained by horizontal arrays of sources and
receivers which have a length longer than the wavelength of the
lowest frequency of the seismic pulse. The spacing between
the elements of the array is less than the wavelength of the
highest frequency in the seismic pulse.
The directivity of the array of sources can be changed
by changing the time delay between the firing of the sources
; in the array.
The foregoing and other objects, features and advan-
tage~ of the invention will be better understood from the
following more detailed description and appended claims.
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Descriptioll of the Drawings
, Figs. 1 and 2 show the marine seismic exploration
system;
Fig. 3 shows the characteristic frequency of a
typical marine seismic source;
Fig. 4 depicts the directivity of a linear array
of seismic sources;
Fig. 5 depicts the response of a 16 element linear
array; and
Fig. 6 is a polar response plot of a linear array.
Description of the Preferred Embodiment
Figs. 1 and 2 show a marine seismic exploration
systemO A vessel 11 traverses a seismic exploration path in
surveying the subsurface formation 12 below the water layer
13. A linear hori~ontal array of seismic sources 14, 15, 16
and others is towed behind the boat by the cable 17. Surface
floats 18, 19, 20 and others help to maintain the equal spacing
between the sourcesO
A hydrophone streamer 21 is also towed behind the
boat. The first hydrophone in the array can be positioned
anywhere from the boat out. Ideally, the first hydrophone is
positioned directly opposite the center of the arxay of sources.
A surface support buoy 22 helps to maintain the proper rela-
; tionship between the hydrophone streamer and the source array.
These are offset one from the other by a distance denoted bythe arrow 23. This offset distance is such that the hydrophones
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in the streamer 21 are sub~ect to horizontally propagated
noise from the source array.
The sources in this array may be any suitable
conventional type of air guns such as the type disclosed in
U.S. patent 3,506,085 to George B. LoperO In such a gun, an
electrical signal operates an electromagnetic valve to allow
high pressure air to be suddenly released from a chamber
within the gun, thereby producing a seismic pulse in the
water. Typically, air guns of this type have a capacity in
the range of 80-200 cubic inches. The characteristic frequency
of the seismic pulse produced by a typical air gun is shown in
; Fig. 3, which depicts the amplitude of the acoustic seismic
pulse as a function of frequency. As depicted in Fig. 3, the
lowest frequency of the seismic pulse is 10 Hz., the highest
frequency is 100 Hz., and the predominant frequency is 30 Hz.
It is desirable to make both the source array and the hydro-
phone array vertically di~ective to discriminate against
horizontally propagating noise, and to maximize the response
to the hydrophones to the broad band frequency content of the
seismic pulse such as that depicted in Fig. 3. Prior art
attempts to maximize the response of hydrophones to the
seismic pulse have configured the source array to obtain
maximum response at the predominant frequency, for example,
at 30 ~z. of the seismic pulse depicted in Fig. 3.
In accordance with this invention, the length 2~ of
` the array, as depicted in Fig. 1, is longer than the wave-
length of the lowest frequency of the seismic pulse. For
example, when using air guns having a characteristic frequency
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depicted in Fig. 3, and assuming a water velocity of 5,000
feet per second, the array is substantially longer than
510 = 500 feet.
Further, in accordance with this invention, the
spacing between the elements of the array, denoted by the
arrow marked 25 in Fig. 1, is less than the wavelength of the
highest characteristic frequency in the pulse. Again, refer-
ring to the example of the air gun having a characteristic
frequency depicted in Fig. 3, the spacing is less than
~ = 50 feet.
As one example of the practice of this invention,
the total length of the array is 800 feet which is substan-
tially longer than 500 feet. The array is made up of 16 guns
_ spaced at 50 foot intervals.
The hydrophones in the streamer 21 may have the
same spacing and array length. The reasons why such arrays
provide vertical directivity and good response over the broad
; band of the seismic pulse will be understood from the following.
Directive arrays can be described with reference to
Fig. 4. The directivity of the array is specified by aT which
is the time required for a seismic pulse to travel from the
source 26 to the point 27. Stated another way, it is the time
re~uired for a seismic pulse to travel from a source at one
end of the array to the plane wave front of the pulses from
other sources. ~T is given by:
T = n ( T -
W
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In the foregoing, n is the number of elements of the array,
QX is the spacing between the elements, ~ is the angle between
the vertical and the path of the pulse, Vw is the velocity of
the seismic pulse in water and I is the time delay between the
S time of firing each source. The frequency of the seismic pulse
is l/T. For the horizontal array of sources under considera-
~ion, I = 0 to obtain maximum vertical directivity.
The steady state response of the geophone streamer
to such a linear array of sources is given by: -
R~QTT~ = Sin ~T/~ (2)
This response is shown in Fig. 5.
In practice, the response of the system is deter-
mined by picking ~ and Vw and computing aT from equation (1).
For the example under consideration, ~T = (800)(~ . With
this ~T, various T's or frequencies are assigned to form the
ratio ~TJT. Fxom the values of ~T/T, the response is deter-
mined from (2) above, or from the response curve in Fig. 5.
A range of such values are tabulated and can be plotted as
polar response curves. Fig. 6 shows such curves. Fig. 6
; 20 depicts the response for a horizon~al linear array having 16
elements spaced at 50 feet with no delay between the sourcesO
The polar response plot depicts the relative strength of
seismic energy at a given frequency propagating at different
angles when compared to that propagating from a point source.
A point source has a polar response plot characterized by the
circle of amplitude 1Ø
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Note that for very large T, the range of values
AT/T will be between 0 and 1. As T decrea~es, ~T/T will
increase beyond 1 and side lobes appear on the polar response
plot (dotted curve). Curves are symmetrical about (~ = 0
~, for case of ~ = 0.
The horizontal arrays of this invention are parti- -
cularly suitable for use where the sources are fired in
separate groups to produce a seismic pulse having a time
domain characteristic representative of the inverse of the
distortion effect caused by reverberation in the water layer.
Such a technique is described in my copending application ~-
in the Netherlands No. 7601413.
While a particular embodiment of the invention has
been shown and described, various modifications are within
the true spirit and scope of the invention. The appended
claims are intended to cover all such modifications.
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