BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to seismic sensor units, and is more particularly but not exclusively concerned with seismic sensor units for land applications. Such sensor units, also called geophones, are used for detecting seismic signals that propagate through the ground. The output signals of numerous such geophones are fed into a seismic survey telemetry system and transmitted to recording and data processing units. Processed seismic data are then presented in form of seismic traces in a seismogram that is useful as information about subsurface stratigraphy. 
     2. Description of Prior Art 
     Most conventional geophones consist of a case that houses and secures at least one internal electric seismic sensor element that senses seismic signals in the ground. Thus, in order to provide a sufficient acoustic and mechanically stable contact with the ground most geophones are equipped with a spike or the like that is attached to the geophone case and planted into the ground. Electric cable is fixed to the case as well, to allow each geophone to be connected to other geophones and to transmit output signals to data acquisition units. The most commonly used seismic sensor elements in conventional geophones are moving coil sensor elements with a large dynamic range and good resolution. But the frequency bandwidth of moving coil geophones is limited due to relatively low frequency resonance, ie in the region of 10 to 30 Hz, and on the high side due to spurious frequency noise. And the performance of such moving coil geophones strongly depends on the verticality of the planted geophone. 
     During seismic data acquisition, a group of several conventional geophones is typically deployed over a certain area and interconnected to each other via cable. Usually the desired seismic signals are contaminated by unwanted noise signals, such as horizontally travelling waves, so-called ground-roll, and random (incoherent) noise such as wind noise, rain noise, scratching of the geophone case by vegetation, geophone cable oscillation, etc. In order to attenuate such noise, the analog output signals of a group of closely spaced geophones are grouped together by adding them into a single analog seismic group signal before being further processed. In total, a very large number of conventional geophones is needed in a seismic survey to realise such an analog method of improving the signal-to-noise ratio of the desired signals. Today, typically  24  geophones are needed per seismic trace. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to overcome the above described drawbacks of conventional geophones and, in a preferred implementation of the invention, to provide a seismic sensor unit that allows the signal-to-noise ratio of the output signals of the seismic sensor units to be improved with, and despite of, a significantly reduced number seismic sensor units per seismic trace. 
     According to the present invention, there is provided a seismic sensor unit comprising: a case that houses at least one electric seismic sensor element and allows said sensor unit to be placed on and acoustically coupled to the ground; a cable, fixed to the case, to allow the supply of power to the sensor unit and to transmit the sensor unit output signal to other seismic sensor units and/or to a signal processing unit; wherein the case also houses a first electronic circuit in which the output signal of the sensor element is digitised, and a second electronic circuit which comprises a telemetry interface module that serves to exchange signals and data with a seismic data acquisition network, said second electronic circuit including a signal filter module and a logic and signal conditioning module which applies calibration coefficients to the sensor unit output signal. 
     The major advantage of a seismic sensor unit according to the invention is that it allows digitised seismic data to be obtained from each individual sensor unit, which data can be digitally filtered in an adaptive manner, so that less sensor units are required per seismic trace. Such processing can be performed remotely, either in electronic boxes along the seismic line, in a central system computer or in a data processing centre. Because of the digital output signals from the seismic sensor units, long analog signal cables, which in conventional seismic systems cause signal disturbances due to loss and noise picked up along the cable, are no longer needed. All this reduces the cost of seismic data acquisition, inter alia by improving acquisition logistics, and improves seismic data quality. 
     Further embodiments of the seismic sensor unit according to the invention make use of feed-back controlled accelerometers as seismic sensor elements. In particular, the use of digital accelerometers that are capable of measuring DC signals allows the determination of the gravity component parallel to the sensor axis. This gravity measurement can be performed during a test period either before or after the seismic data acquisition. Quality control of acquired seismic data can be provided even during the measurement phase in the field. 
     Another embodiment of the seismic sensor unit according to the invention applies recently developed manufacturing technology of electronic components and sensor elements. The major advantage that can be thereby achieved is that the seismic sensor unit will be of a smaller size than geophones available today, and of less weight. Even in view of geophysical aspects, such smaller size of the seismic sensor element is advantageous, because it renders the seismic sensor less sensitive to non-linear noise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from the following detailed description when taken into conjunction with the attached drawings, of which: 
     FIG. 1 is a schematic side view of a first embodiment of a seismic sensor unit according to the invention; 
     FIG. 2 is a schematic detailed block diagram of the sensor element and electronics of the seismic sensor unit according to FIG. 1; 
     FIG. 3 is a detailed block diagram of the sensor elements and electronics of a second embodiment of a seismic sensor unit according to the invention; 
     FIG. 4 is a schematic drawing that illustrates a first implementation of the seismic sensor unit according to FIG. 3; 
     FIG. 5 is a detailed schematic side view of the sensor unit of FIG. 4; 
     FIG. 6 is a schematic drawing that illustrates a second implementation of the seismic sensor unit according to FIG. 3; and 
     FIG. 7 is a perspective side view of the sensor unit of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The seismic sensor unit according to this invention will be described with reference to the drawings of FIG.  1  through FIG.  7 . For simplicity reasons, not all of the wiring inside the seismic sensor unit that interconnects electronic circuits and sensors is shown, since a person skilled in the art will easily understand from the following explanations how to provide such wiring. 
     FIG. 1 shows schematically a preferred embodiment of a seismic sensor unit  10  according to the invention. A case  12  of the sensor unit  10  is shown partially broken-away in order more easily to explain its interior. A spike  14 , well known from conventional geophones, is fixed to the case  12  and serves to plant the sensor unit  10  properly into the ground. 
     Inside the case  12 , a seismic sensor element is shown at  16 . The sensor element  16  illustrated in FIG. 1 as an example is a 1-component seismic sensor element. Although the seismic sensor unit  10  according to the invention is capable of operating with different types of seismic sensor elements, ie conventional moving coil sensor elements, the preferred seismic sensor element is a feed-back controlled accelerometer. The feed-back controlled accelerometer is preferably a silicon micro-machined sensor that allows the measurement of seismic signals down to very low frequencies, ie 3 Hz or lower, which, as explained later, are useful for verifying the verticality of the planted seismic sensor unit  10 . 
     FIG. 1 also illustrates electronic circuits which are mounted on two printed circuit boards  18 ,  20 . The electronic circuits will be described later in detail, together with their functions. A seismic cable  22  is fixed to the case  12  and connects the seismic sensor unit  10  to other seismic sensor unit and/or to a seismic system network which, for simplicity reasons, is not illustrated in FIG.  1 . Attached to the cable  22  is a microphone  24  which is useful for determining acoustic noise that may affect the seismic signals. In order to allow compensation for temperature changes that may cause drift in the electronics, a temperature sensor  26  is used to determine the temperature inside the case  12 . 
     FIG. 2 illustrates schematically details of the electronics inside the case  12  of the seismic sensor unit  10  of FIG.  1 . The analog output signal of the 1-component feed-back controlled accelerometer  16  is passed through a feed-back control electronic module  30  that controls in particular the centring of the seismic mass of the accelerometer  16  during vibrations, ensures linearity of the accelerometer, and separates seismic (acoustic) signals from DC signals with a frequency below 3 Hz. After having passed through an analog low pass filter (not shown), the seismic signals are fed into a multiplexer  32  and then passed to an analog/digital converter (ADC) referred to by  34 . Preferably, this ADC  34  is a sigma-delta converter that converts at very high sampling rates. 
     Output signals of the feed-back controlled accelerometer  16  with a frequency equal or less than about 3 Hz are treated as DC signals. Such DC signals allow a determination the local gravity effect at the planted seismic sensor unit  10 . The measured gravity signal indicates whether the seismic sensor unit  10  is properly planted in the desired vertical position, or whether its inclination is still acceptable or not. As mentioned earlier, the feed-back control electronic module  30  separates the DC signals from the usual seismic signals from the seismic sensor  16  and passes them to an auxiliary multiplexer  36  from where the signals are applied to an auxiliary analog/digital converter (ADC)  38 . The ADC  38  may also be a sigma-delta converter. 
     Output signals of both ADCs  34  and  38  are further processed in a block  40  of electronic modules that comprises at least a decimator  42 , a low pass signal filter module  44 , a logic module  46  with an attached data storage module, and a telemetry module  48 . As is well known, the sigma-delta converter used as the ADC  34  has a high sampling rate at low accuracy. The decimator  42  is complementary to the sigma-delta converter that operates as a modulator at a high sampling rate, ie at 400 kHz, with typically one bit resolution. The decimator  42  processes digitally the oversampled bit stream to generate high accuracy samples of typically 24 bits at lower rate, ie 24 ms. 
     A digital low pass filter to attenuate noise is then applied by the low pass filter module  44  to the signals from the decimator  42 . The signals then pass to the logic module  46 , which applies calibration coefficients that are stored in its attached data storage means, preferably an EEPROM, and corrects the signals therewith. The telemetry module then conditions the signals and feeds them into the seismic data network system, which in FIG. 2 is symbolised by a seismic data bus  50 . The seismic data bus  50  is connected to further seismic data recording and processing units that are not illustrated. 
     In FIG. 2, a test signal generator is shown at  52 . This test signal generator  52  is capable of generating various test signals which serve to verify the functionality and performance of each module shown in FIG.  2 . The test signals from the test signal generator  52  can therefore be fed either to the ADC  34  via the multiplexer  32  or via the multiplexer  32  back to the feed-back control module  30 . 
     All above described electronic modules  30 ,  32 ,  34 ,  36 ,  38 ,  40 ,  52  are designed and manufactured using large scale integrated circuit technology, and more particularly as a minimum number of ASICS. This leads to a very small volume for the modules and allows them to be mounted on one single printed circuit board, as represented in FIG. 1 by the board  18 . 
     A power regulator  54 , preferably including a DC/DC converter or voltage regulator, serves to supply the electronics with power. Power provided by a power line  56 , which together with the data bus line  50  - represents the seismic cable  22  (see FIG.  2 ), is stabilised by the power regulator  54  and distributed to the electronic modules inside the case  12 . For simplicity, the wiring necessary for power distribution is not illustrated in FIG. 2, because this will be evident to a person skilled in the art and is not essential for an understanding of the present invention. 
     FIG. 2 shows the microphone  24  and temperature sensor  26 , which have been already described above in relation to FIG.  1 . FIG. 2 illustrates that the output signal of the microphone  24  is passed into the same multiplexer  32  as the seismic signal from the seismic sensor element  16 . Each of the microphone  24 , the seismic sensor element  16  and the test signal generator  52  is activated separately from the others. The output signal of the temperature sensor  26  is fed into the auxiliary multiplexer  36 , as is the DC signal from the feed-back controlled accelerometer  16 . 
     In FIG. 3, a second embodiment of a seismic sensor unit according to the invention is illustrated, specifically an embodiment designed for a 3-component seismic element. Most of the electronic modules are the same or at least similar to those which have been already described in relation to FIGS. 1 and 2. The electronics of the embodiment schematically shown in FIG. 3 are such that they are suitable for a 3-component seismic sensor, which is preferably a temperature-compensated 3-component feed-back control accelerometer made as a silicon micro-machined device. 
     In the following description, the three components of the seismic sensor element  16  of FIGS. 1 and 2 are referred to as  16   x ,  16   y  and  16   z . As described above in relation to FIG. 2, the output signals of the components  16   x ,  16   y  and  16   z  of the seismic sensor element  16  are fed into the related circuitry for each component&#39;s signal. Thus, the feedback control modules  30   x ,  30   y ,  30   z  control the performance of each seismic sensor component  16   x ,  16   y ,  16   z . And as in the embodiment of FIG. 2, each component signal is then further processed in related multiplexers  32   x ,  32   y ,  32   z  and digitised in related analog/digital converters (ADCs)  34   x ,  34   y ,  34   z . Preferably the ADCs are sigma-delta converters with the same features as already described in relation to FIG.  2 . 
     As in the embodiment of FIG. 2, the auxiliary multiplexer and the auxiliary ADC are referred to as  36  and  38 . Similarly, the DC signals of each seismic sensor element component  16   x ,  16   y ,  16   z  are passed to the auxiliary multiplexer  36  and then to the auxiliary ADC  38 . 
     As a further modification to the embodiment shown in FIG. 2, a 3-component magnetometer  58   x ,  58   y ,  58   z  is included in the seismic sensor unit  10  of FIG.  3 . The magnetometer is preferably a flux gate magnetometer with an associated electronic module that generates an AC flux across the magnetometer sensors and determines the DC offset of the hysteresis loop. This offset signal is proportional to the local earth magnetic vector. The performance of the magnetometer is controlled by a magnetometer electronic control module  60 , from which the measurement signals of the magnetometer components are passed into the auxiliary multiplexer  36 . The orientation of the axis of the magnetometer components  58   x ,  58   y ,  58   z  is the same as the orientation of the related components  16   x ,  16   y ,  16   z  of the feed-back controlled accelerometer, in other words, the axis of the accelerometer component  16   x  is parallel to the axis of magnetometer component  58   x , and the same for the respective y and z axes. Together the magnetometer signals and the gravity related DC signals of the seismic sensor accelerometer are very useful to determine (and compensate for) the orientation of the seismic sensor unit  10  in view of inclination and azimuth. 
     The seismic signals from the ADCs  34   x ,  34   y ,  34   z  and the magnetometer signals and the DC gravity related accelerometer signals from the auxiliary ADC  39  are passed into the block of electronics  40  that has been already described in relation to FIG.  2 . The various electronic modules  42 ,  44 ,  46 ,  48  (see FIG. 2) in the block  40  will then apply filtering and conditioning to the signals as also explained above. Preferably separate electronic modules x, y, z of the decimator  42  as well as of the digital low pass filter  44  perform the conditioning of the signal components x, y, z. Other modules similar to those illustrated in FIG. 2, eg a test signal generator, are not shown in FIG. 3 for simplicity, but it will be apparent to a person skilled in the art how such a test signal generator  52  can be connected to the seismic sensor element components  16   x ,  16   y ,  16   z.    
     Again, the electronic modules  32   x ,  32   y ,  32   z .  34   x ,  34   y ,  34   z ,  36 ,  38 ,  40 ,  60  are designed and manufactured using large scale integrated circuit technology, and more particularly as ASICS. 
     FIGS. 4 and 5 show how a seismic sensor unit  10  with a 3-component seismic sensor element  16   x ,  16   y  (not shown) is mechanically constructed. Inside the case  12  of the seismic sensor  10 , electronic boards  18 ,  20  are arranged between the seismic sensor element components  16   x ,  16   y ,  16   z  and the magnetometer components  58   x ,  58   y  and  58   z  (not shown). If the seismic cable  22  is fixed to the case  12  as shown in FIG. 4, the magnetometer components  58   x ,  58   y  and  58   z  should be mounted at the opposite end of the case to the cable  22 . Two spike  14 . 1 ,  14 . 2  are fixed to the case  12  as shown in FIG.  5 . The advantage of this two spikes embodiment is that it ensures a mechanically stable planting in the ground. 
     In FIGS. 6 and 7, another embodiment of a seismic sensor unit  10  according to the invention is shown, which embodiment comprises a 3-component seismic sensor element  16  (of which only component  16   y  is shown) and a 3-component magnetometer  58  (of which only component  58   y  is shown). The case  12  of this sensor unit has a spherical part  62  that houses the magnetometer  58 , which again is mounted at the opposite end of the case to the seismic cable  22  because of the current in the cable. Furthermore, the magnetometer  58  is mounted opposite to the seismic sensor element  16  in order to limit potential magnetic perturbations that may be caused by magnetic or metal parts inside the accelerometer  16 . The case  12  is equipped with several small spikes  64 , as shown, to ensure sufficient acoustical coupling to the ground without a conventional (and rigid) planting action. This type of case  12  also supports simplified or automatic deployment of the sensor units  10 , because the case  12  always tends to turn itself into the best position on the ground.
A seismic sensor unit ( 10) comprises a case ( 12) containing electronic circuits ( 18, 20) that serve to digitise and digitally process the output signals of a seismic sensor element ( 16) inside the case ( 12). A first digital filtering of acquired signals can be provided at sensor unit level before the data are further fed into a seismic network and further processed.