Patent Publication Number: US-7898903-B2

Title: Combined probe and corresponding seismic module for the measurement of static and dynamic properties of the soil

Description:
TECHNICAL FIELD 
     This disclosure relates to a combined seismic probe and corresponding seismic module for the measurement of static and dynamic proprieties of the soil. 
     BACKGROUND 
     A known probe  1  for measuring in situ the deformation moduli of the soil layers, commonly referred to as a Flat Dilatometer, is known from U.S. Pat. No. 4,043,186. This probe  1 , shown in  FIG. 1 , comprises a dilatometer blade  2  having inside a pressure chamber in communication with the outside via an opening sealed by a thin circular expandable steel membrane  3  mounted flush on one face of the blade (or equipped with two membranes, one on each face of the blade). The probe is forced vertically into the ground by push rods  4  connected above the blade  2 . The dilatometer blade  2  also comprises electrical contacts acting in response to the movement of said metal membrane. The blade  2  is connected to the surface by an internally wired gas conduit  5 , 6  comprising a gas conduit  5 , for example a plastic tube, containing a single wire  6 , which runs inside the push rods  4 . The gas conduit  5  is connected, at the soil surface, to an external circuit comprising an unit for introduction and removal of compressed gas to and from the chamber dilatometer blade  2 , while the single wire  6  allows the electrical communication from the blade  2  to the external circuit. At selected depths the blade  2  is stopped and the membrane  3  is inflated by feeding gas pressure to the blade  2 . At each depth the user determines the pressure Po which is the pressure causing the initial lift-off of the membrane, and subsequently the pressure P 1  which is the pressure producing a central displacement of the membrane by a predetermined amount, generally 1.1 mm. The instants at which Po and P 1  have to be taken are signaled by the opening or the closure of a circuit, determined by the position of the membrane, which works like an ordinary electrical switch. The two poles of the switch are connected to the surface by the live wire inside the gas conduit and the electrical earth i.e. the pushrods. On the surface, a battery and a buzzer (emitting a sound when the circuit is closed) complete the circuit. The two pressures Po and P 1  are then interpreted to derive soil parameters. 
     The known Flat Dilatometer probe has been increasingly used in recent decades. However the Flat Dilatometer determines only static soil parameters, while today there is also a growing interest in seismic parameters, in particular in the shear wave velocity Vs and in the initial shear modulus Go (derivable from Vs). 
     Various methods are currently available for determining the Vs profile. Among these, frequently used methods are the Cross-Hole and the Down-Hole method, carried out in specially made dedicated boreholes. However these methods require additional field work, hence the global cost and time are substantially higher compared with a situation wherein static and seismic parameters are obtained from just one sounding with the same probe. 
     This is the reason why research efforts have been carried out in the past (Hepton “Shear Wave Velocity Measurements during Penetration Testing”, Proc. Penetration Testing in the UK, ICE1988, pages 275-278, and G. Martin and P. Mayne “Seismic Flat Dilatometer Tests in Piedmont Residual Soils” Geotechnical Site Characterization 1998 Balkerna, Rotterdam) to combine the Flat Dilatometer with a seismic module for obtaining at the same time the static and seismic parameters. 
       FIG. 2  shows a known experimental composite probe equipped with a seismic module  7 . The seismic module  7  comprises a tubular element  4   a , for example a steel tube, connected above the dilatometer blade  2 . Inside this tubular element  4   a , two receivers  8 ,  9 , for example geophones or accelerometers are placed, spaced typically 0.5 m or 1 m apart. 
     Each receiver  8 ,  9  is connected to the surface by a respective wire  10 ,  11 . Each of these wires  10 ,  11  runs inside the push rods  4  in parallel to the internally wired gas conduit  5 , 6 . Both the internally wired gas conduit  5 , 6  and the seismogram wires  10 ,  11  reach the surface and are connected to an external circuitry. 
     The composite probe  12 , formed by the seismic module  7  and the dilatometer blade  2 , is forced vertically into the soil by push rods  4  connected above the tubular element  4   a  of the seismic module  7 . At the desired depths the probe  12  is stopped and the operator can either perform measurements of static soil parameters—as described above—or can perform a Vs measurement. 
     At the depths where the Vs measurements have to be carried out, the probe  12  is stopped and a seismic wave W is generated at ground surface by a source, often a pendulum hammer which hits horizontally a parallelepiped anvil. The seismic wave W propagates downwards and reaches first the upper receiver  8 , then the lower receiver  9 . The delay between the first and second seismogram is generally determined using the well known cross-correlation algorithm. 
     Once the delay is known, the shear wave velocity Vs is obtained as the ratio between the easily calculated difference in distance between the source at the surface and the two receivers and said delay. Vs may then be converted into Go, the initial soil shear modulus, by using the theory of elasticity formula Go=ρVs 2 . 
     When the combined probe  12  is used for static measurement, the Flat Dilatometer works as previously described and is connected to the surface by its internally wired gas conduit  5 , 6 . 
     The seismic module  7  transmits in analog form to the surface the electrical seismograms generated by the two receivers  8 , 9  via wires  11 ,  10 . The seismograms are analyzed at the surface by an oscilloscope to determine the delay. 
     Such combined probes  12  have produced interesting research results. In particular the obtained results have been the starting point of studies aimed at combining the low strain shear modulus (from the Seismic test) and the operative modulus (from the Flat Dilatometer) for defining the decay curves of modulus versus strain, necessary to perform non linear analysis of the soil deformation under load. 
     However, the known combined probe  12  has a number of serious practical drawbacks for industrial use in the field of the soil investigations. 
     One inconvenience is that the wires  10 ,  11  transmitting the seismograms in an analog form act like antennas and pick up electrical disturbance due to traffic, motors, electrical lines, telecommunication lines and the like. This inconvenience is especially grave at large depths, where the seismograms are weak—due to the distance from the energizing source—and the electrical noise can obscure the signal running on the wires  10 ,  11 . 
     An even more serious practical inconvenience is the presence of multiple cabling. The presence of more than one cable, in particular either the internally wired gas conduit  5 , 6 , and the wires  10 ,  11 , enormously complicates field testing, as it is well known to experienced operators. The cables  5 ,  6 ,  10 ,  11  have to be threaded inside the push rods  4  and frequently manipulated. The risk of garbling and mixing up two (or more) cables is high, resulting in considerably slower operations and considerably higher overall costs. 
     In the case of deep investigations in the ground, several cables  5 , 6 ,  10 ,  11  must be must be connected in sequence to obtain the necessary length. The junctions become numerous and the multiple joints, pneumatic and electrical are highly complicated and voluminous. 
     In the special case of offshore investigations, which are very important to industry, the use of a multiplicity of wire or cables is virtually impossible. In fact in many offshore configurations, the dilatometer blade  2  connected to the internally wired gas conduit  5 , 6  is suspended by a rope that is even difficult to handle with just a single cable or conduit because the suspended dilatometer blade  2  can rotate, thereby twisting the internally wired gas conduit  5 , 6 , while the rope and the internally wired gas conduit  5 , 6  must freely slide longitudinally independently of each other, in order to allow the insertion of the dilatometer blade  2  in the soil at the bottom of the sea. It is a well known fact that operators are in many cases obsessed by the problems posed by the presence of wires. 
     The overcoming of these inconveniences, in particular the multiple wire problem and advantageously the antenna effect, are solved by the embodiments of the present disclosure. 
     SUMMARY 
     In a first embodiment disclosed herein a combined probe includes a dilatometer probe, a gas conduit coupled to the dilatometer probe for providing a gas connection between the dilatometer probe and an external gas source, a wire located in the gas conduit for providing an electrical connection between the dilatometer probe and an external circuit, and a seismic module coupled to the wire located in the gas conduit to provide an electrical connection between the seismic module and the external circuit. 
     In another embodiment disclosed herein a seismic module comprises a tubular element, a gas conduit located within the tubular element, and a wire located in the gas conduit to provide an electrical connection between the seismic module and the external circuit, wherein the wire and the gas conduit are adapted for coupling to a dilatometer probe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the features and many of the attendant advantages thereof will be readily understood by reference to the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  shows a schematic view of a dilatometer probe in accordance with the prior art. 
         FIG. 1   a  shows a schematic side view of the dilatometer probe of  FIG. 1  with the membrane in an unexpanded position. 
         FIG. 1   b  shows a schematic side view of the dilatometer probe of  FIG. 1  with the membrane in an expanded position. 
         FIG. 2  shows a schematic view of a combined probe in accordance with the prior art. 
         FIG. 3  shows a schematic view of a combined probe in accordance with the present disclosure. 
         FIG. 4  shows a schematic view of an embodiment of a seismic module of the combined probe in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings wherein like reference numerals describe identical or corresponding parts throughout the several views, and more particularly to  FIG. 3 , a combined probe  130  comprises a dilatometer probe  20  having inside a pressure chamber which communicates with the outside via at least one opening sealed by a membrane  30 , gas being feedable into and removable from the pressure chamber to cause the membrane to move. For example, the gas can be air or nitrogen. 
     The combined probe  130  also comprises an internally wired gas conduit ( 50 ,  60 ) providing electrical and pneumatic connection between the dilatometer probe to an external circuit (not shown). In particular, the internally wired gas conduit comprises a gas conduit  50 , for example a plastic tube, containing a wire  60 . The gas conduit  50  permits the compressed gas to flow from the external circuit on the soil surface to the dilatometer probe  20 , while the wire  60 , for example a single wire, permits the exchange of electrical signals between the dilatometer probe  20  and the external circuit. 
     The combined probe  130  comprises a seismic module  70  that comprises a tubular element  41 , for example a rigid tubular element  41 , connectable to the dilatometer probe  20 . Advantageously, the rigid tubular element  41  is a steel tube. Inside the tubular element  41 , at least a seismic transducer element  80 ,  90 , an electronic board  100  connected to the receivers  80 ,  90  are fixed. The internally wired gas conduit ( 50 ,  60 ) is inserted in the tubular element  41  before reaching the external circuit. 
     The electronic board  100  comprises a transmitter  110  to exchange the electrical signals between the electronic board  100  and the external circuit. 
     In one embodiment, the transmitter  110  is coupled to the wire  60  in order to exchange signals with the outside of the seismic module  70 . 
     With the combined probe  130  it is possible to carry out both static measurements of the soil parameters using the dilatometer probe  20  and dynamic/seismic measurements of the soil parameters using signals originating from the seismic transducer element  80 ,  90 . 
     In particular, both the dilatometer probe  20  and seismic module  70  use the internally wired gas conduit ( 50 ,  60 ) in order to exchange information with the outside of the combined probe  130 . 
     Advantageously the only connection between the dilatometer probe  20  or the seismic module  70  and the external circuit is constituted by the internally wired gas conduit ( 50 ,  60 ), thereby avoiding the problems caused during field operations by the presence of multiple cables. 
     In order to obtain more detailed dynamic measurements of the soil parameters, a second seismic transducer element  80 ,  90  can be provided in the seismic module  70 . The seismic transducer elements  80 ,  90  are receivers, for example geophones or accelerometers, spaced typically 0.5 m or 1 m apart. 
     Advantageously, the seismic module  70  and the dilatometer probe  20  are forced vertically into the soil by means of push rods  40  that are connected above the rigid tubular element  41 . 
     In one embodiment, the electronic board  100  also comprises: 
     a receiver which receives signals originating from the seismic transducer element, and 
     an analog/digital converter for amplifying and transforming the analog signals from the seismic transducer element  80 ,  90  into a digital signal. 
     In the embodiment shown in  FIG. 3 , the transmitter  110  is contactless and comprises, for example, a magnetic head with a C-shaped profile, in which the internally wired gas conduit ( 50 , 60 ) runs between the free arms of the magnetic head. 
     In another embodiment shown in  FIG. 4 , the transmitter  110  comprises a wire  111  having a first end which is electrically connected to the electronic board  100 , a second end which is electrically connected to the wire  60 . The second end of the wire  111  is located in a pneumatic adaptor  112  which is sealedly closed with the internally wired gas conduit ( 50 ,  60 ). So the connection between the wire  111  and the wire  60  is sealedly inside the pneumatic adaptor  112 , in order to permit the flow of compressed gas in the gas conduit ( 50 ,  60 ). 
     In other words, the second end enters in the internally wired gas conduit through the pneumatic adaptor so that compressed gas can freely flow in the internally wired gas conduit ( 50 ,  60 ). After that the conductor  111  is connected to the wire located in the gas conduit  50 . 
     Advantageously, airtight connectors  120  are provided at the ends of the internally wired gas conduit ( 50 , 60 ) of the seismic module  70  in order to permit the connection to the dilatometer probe  20  and to a further internally wired gas conduit ( 50 , 60 ) of a dilatometer probe  20 . 
     Also advantageously, the seismic module  70  can include geophones, accelerometers, inclinometers, video cameras, pressure transducers, visual sensors, chemical detectors connected to the electronic board. The electronic board  100  can also be formed by acquisition unit and a processor. 
     A seismic module  70 , as for example those shown in  FIGS. 3 and 4 , comprises: 
     seismic transducer elements  80 , 90 , 
     an acquisition unit and a processor  100  for signals originating from the seismic transducer elements, and 
     transmitter  110  is connected to the acquisition unit and processor  100 . 
     Seismic modules  70  are suitable to contain an internally wired gas conduit ( 50 ,  60 ) for electrical and pneumatic connection, and are suitable to be connected to a dilatometer probe  20 , hence define a combined probe  130 . As previously described, the dilatometer probe  20  has inside a pressure chamber communicating with the outside via at least one opening sealed by a membrane  30 . The internally wired gas conduit ( 50 ,  60 ) is connected and sealed to the pressure chamber, while a wire  60  located in the internally wired gas conduit ( 50 ,  60 ) is electrically connected to the dilatometer probe  20 . 
     In the seismic module  70 , the transmitter  110  is coupled to the wire  60  to permit the exchange of electric signals between the acquisition unit and the processor and the outside of the seismic module. 
     The combined probe  130  operates, for example, in the following manner. The outputs of the receivers  80 ,  90 , rather than being sent directly to the surface as in the prior art, are sent to the electronic board  100  inside the seismic module  70 . The electronic board  100  processes the signals, for example it amplifies and digitizes them. Then, rather than using additional wires to transmit the signals to the surface, the digitized signals are conveyed to the surface via the inner wire  60  used by the dilatometer probe  20 . The digitized signals are decoded by the surface unit and analyzed to determine the delay, from which the shear wave velocity is computed. 
     Advantageously the digitization at depth avoids the antenna effect that would disturb the signals if they were transmitted directly from the transducers to the surface by wires in an analog form. 
     The double use of the inner wire  60  for both static and seismic measurements, enabling alternatively the dilatometer probe or the seismic module, is possible since the two tests are carried out at different times. The combined probe  130  makes the combined tool much simpler and economical to use. 
     Advantageously, the combined probe  130  can use the same internally wired gas conduit ( 50 , 60 ) as the dilatometer probe, this arrangement having various advantages. Users of the Flat Dilatometer probe do not need to procure additional cables and the manufacturer does not need to manufacture and have in stock different types of cables and joints (beneficial also from the ecological viewpoint). 
     Advantageously with the combined probe  130 , the detected signals can transmit digitally, via the same inner wire  60 , even those signals generated by other sensors included in the seismic module  70 . 
     The presence of just one internally wired gas conduit ( 50 , 60 ) permits very quick disassembly of the combined probe  130  if the seismic module  70  is removable. Thus, if only static measurements have to be carried out, the dilatometer probe and the seismic module can be quickly separated and the dilatometer probe be used in the usual non-seismic mode. 
     The modularity of the composite probe means that owners of the Flat Dilatometer probe, for example described in the U.S. Pat. No. 4,043,186, wishing to also carry out seismic measurements, do not need an integrated new probe but need only add-on the seismic module  70  of this disclosure to upgrade their Flat Dilatometer probe.