Acoustics – Geophysical or subsurface exploration – Seismic source and detector
Reexamination Certificate
2001-10-01
2004-09-07
Fletcher, Marlon T. (Department: 2837)
Acoustics
Geophysical or subsurface exploration
Seismic source and detector
C181S122000
Reexamination Certificate
active
06786297
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to systems deployed on-site on land or at sea for the acquisition of geophysical data.
2. Description of the Related Art
These systems use an assembly of sensors, linked by electrical cables to casings whose role is to process the data emanating from the sensors, in particular by digitizing the data and transmitting them to a central processing unit to which the casings are also linked by electrical cables. These casings can also comprise means making it possible to test the operation of the sensors and the digitizing of the data.
The known systems are generally designed according to one of the following two architectures, which will now be explained with reference to FIGS.
1
and
2
:
monotrack architecture (represented in FIG.
1
),
multitrack architecture (represented in FIG.
2
).
FIG. 1
is a diagram representing a monotrack architecture. In this diagram, the geophysical data acquisition system S comprises a plurality of tracks T(i), each of which consists of an assembly of geophysical sensors.
Such tracks T(i) are well known and conventionally consist of n identical modules which each link in series or in parallel m geophysical sensors such as geophones whose analog output signal characterizes the response of the subsurface strata to the signal emitted following the activation of one or more seismic sources.
The monotrack system S also comprises casings B(i) for digitizing the analog data emanating from the sensors of each track, and transmitting these data to storage means (not represented in the figure). Each track T(i) is thus linked to a respective casing B(i) by a cable
10
connected to a port P(i) of the casing, said cable conveying the analog data emanating from the sensors of the track T(i).
The casings B(i) comprise means for digitizing these analog signals, and for transmission to the storage means by way of a cable C which links the casings in series.
The cable C is composed of sections C(i) conveying the digital signals emanating from the casings B(i) as well as the electrical power supply required for the operation of these casings. Each section C(i) is furnished at each of its two ends with a connector
20
for coupling R with a casing. Each casing B(i) therefore comprises in addition to its port P(i) two connectors for cooperating with the connectors
20
of two cable sections.
The diagram of
FIG. 2
represents a so-called “multitrack” or “N-track” system S′, according to the second type of architecture commonly employed.
The multitrack system S′ comprises casings B′(j) for digitizing and transmitting data, each casing being linked to N tracks T(i) (4 tracks for each casing in the instance of the system represented here, but N-track systems in which N is equal to 6 for example are also commonly used). Each track is for its part linked to a single casing, by way of a cable
10
conveying the analog data emanating from the sensors of the track.
An important difference as compared with the monotrack system S represented in
FIG. 1
is that in the instance of the multitrack system, the cables
10
for transmitting analog data are linked to the casings B′(j) not directly by a port, but by way of a main cable C′ to which the casings are linked in series and to which the cables
10
are coupled by so-called take-outs E(i) as they are widely known in the art.
The cable C′ transmits, like the cable C of the monotrack system of
FIG. 1
, the digital data emanating from the casings to storage means, not represented in the figure.
An N-track system thus comprises N times fewer casings than tracks, each interval between two consecutive casings comprising N take-outs of which the first N/2 are linked to a first of the two casings, the other N/2 take-outs being linked to the second casing.
The cable C′ of the multitrack system S′ is more complex that the cable C of the monotrack system of FIG.
1
. This cable C′ thus comprises inside a single sheath:
the extensions of the cables
10
for routing the analog data emanating from the tracks of sensors to the corresponding casing,
conductors for transmitting digital data,
at least one conductor for supplying power to the casings.
The casings B′(j) are linked to the cable C′ by connectors of the casing cooperating with matching connectors
20
′ of the cable C′ so as to constitute couplings R′.
In the two known architectures described hereinabove, the distance between two tracks T(i) is typically of the order of 50 meters. This distance is also that which separates two consecutive casings of a monotrack system, while the casings of an N-track system are separated by around (N×50) meters.
These two architectures each comprise advantages and drawbacks, which may be summarized as follows:
Advantages of monotrack
Advantages of multitrack
architecture (FIG. 1)
architecture (FIG. 2)
Quality of the signal
Reduction in the number of
transmitted: the analog
man cable/casing couplings
lines (from the track to
(divided by 4 in the example
the casing) are short and
of
FIG. 2
; by N in the general
insulated from one
instance of a system with
another.
N tracks) and in the associated
Flexibility of deployment
cost.
in the field (the cable C
Reduction in the number of
is simple and lightweight
casings and in the associated
to handle, and it is there-
cost.
fore easy to tailor it to
the local topography (so
as to bypass obstacles for
example)
Simplicity of the main
cable 10, and of the
connectors of this cable
with the casings B(i).
Reduced size of the
casing B(i) and of the
cable C.
Drawbacks of monotrack
Drawbacks of multitrack
architecture (FIG. 1)
architecture (FIG. 2)
Large number of cable
Lack of flexibility (system
C/casing couplings (2
whose basic element is an
connections per track).
assembly of N tracks).
Number of casings (1 per
Weight and complexity of
track); associated costs
the cable C′.
of equipment and handling.
Problems regarding the
quality of the analog
signals received by the
casings B′(j): several
neighboring strands
contained within the same
sheath convey low-level
detectable analog signals,
this possibly giving rise
to crosstalk. Moreover,
the sensitive analog links
between the sensors of a
track and their associated
casing may be lengthy (for
example 125 meters for a
6-track system).
The two architectures described hereinabove have moreover common drawbacks:
Firstly, the number of couplings R or R′ is sizeable, even if this number is reduced in the instance of a multitrack system. Since the data acquisition installations can be moved in the field, one and the same piece of hardware comprising the tracks and the casings is successively deployed and gathered up at various locations, this involving very many operations for making and undoing the multiple couplings of the system. It is therefore understood that this large number of couplings is especially detrimental in terms of cost of labor and timescales.
Another drawback common to both types of system is that each of the casings which they employ comprises two connectors for coupling with a main cable. The presence of these connectors on the casing constitutes a sizeable obstacle to the miniaturization of the casing, while present-day technological developments make it possible to substantially reduce the bulkiness of the other components of the casing. It would nevertheless be advantageous to reduce the size of the casings, which at present constitute voluminous elements of the systems and may be an impediment to the laying and gathering operations.
A third drawback common to present-day systems stems from the fact that it is sometimes necessary to supplement the couplings between the main cable and the casings with load take-up devices, such as portions of tension cables, one end of which is fixed to a part of the electrical cable close to the casing and the other end of which is mounted, in a removable or nonremovable manner, on the casing itself.
This arrangement may be necessary when the
Blakely & Sokoloff, Taylor & Zafman
Fletcher Marlon T.
Sercel
Warren David S.
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