associated method

In a first aspect, the present invention concerns an assembly for acquiring a physical parameter in a granular media.

The acquisition is for example carried out in a sandy environment such as a sand desert or a sandy beach. The granular media is therefore sand.

The assembly is in particular intended to carry out a seismic survey.

Such a geophysical survey is for example conducted by placing an array of seismic sources into the ground and by deploying seismic probes able to record reflections, refractions or direct waves of seismic signals produced by the successive sources on the different layers of the ground.

For large surveys, wireless probes may be used. Such seismic probes record the seismic signal and wirelessly send the recorded data to a control unit located remotely from the seismic probes, for example in a control center. The communication between the seismic probes and the control unit is generally made in real time. Therefore, quality assurance and quality control of the recorded data may be performed during the seismic acquisition and decision to repeat some measurements may be taken rapidly in case of low quality measurements, for instance in case of noisy data.

In sandy environments, sandstorms may rapidly change the topography of the ground and induce sand accumulations (several tens of centimeters) in some areas. An additional layer of sand may cover the seismic probes. Consequently, the communication between the seismic probes and the control unit may be affected or even lost. This may compromise the good execution of the seismic survey and may require an intervention of a crew. Moreover, even if the location of each seismic probe is known, the recovery of the seismic probe may be hazardous and the seismic probe may be lost.

One aim of the invention is to provide an assembly for acquiring a physical parameter which allows overcoming the above-mentioned drawbacks. In particular, one aim of the invention is to provide an assembly which allows improving wireless seismic surveys by reducing the communication issues between the seismic probes and the control unit while facilitating the recovery of the probes when the seismic survey is completed.

To this aim, the subject-matter of the invention is an assembly for acquiring a physical parameter in a granular media comprising:

- at least one probe comprising a housing receiving a control unit and at least one geophysical sensor connected to the control unit, the probe being intended to be installed at least partially in the granular media, - at least an excitation apparatus configured for generating an upward displacement of the probe with respect to the granular media.

The assembly according to the invention may comprise one or more of the following features, taken solely or according to any potential technical combination:

- the excitation apparatus comprises a vibration unit connected to the control unit and mechanically connected to a wall of the housing, the vibration unit being configured for vibrating the probe in the granular media;

- the excitation apparatus further comprises:

- a receiver connected to the control unit and configured for receiving at least a signal, and

- a remote emitter, located remotely from the probe, configured for emitting the signal;

- the signal is an excitation signal, the remote emitter being configured for emitting the excitation signal, the control unit being configured for activating the vibration unit when the receiver receives the excitation signal;

- the signal is a periodic control signal, the remote emitter being configured for emitting the periodic control signal at a predetermined time period, the receiver being configured for receiving the periodic control signal, the control unit being configured for activating the vibration unit in case of loss of reception of the periodic control signal after a duration longer than the predetermined time period;

- the control unit is configured for deactivating the vibration unit in case of restoration of the reception of the periodic control signal by the receiver;

- the vibration unit comprises an eccentric rotating mass;

- the vibration unit has a vibration frequency, the vibration frequency being comprised between 0.5 Hz and 100 Hz;

- the housing comprises an external wall presenting a spherical shape or a cylindrical shape or an egg-shape;

- the probe comprises at least one part made in a ferromagnetic material, the excitation apparatus comprising a magnetic apparatus configuring for generating a magnetic field in the granular media and to attract the probe, the magnetic apparatus being located remotely from the probe;

- the magnetic apparatus comprises a rotating magnetic plate intended to rotate with respect to the granular media, above the granular media, according to an axis substantially perpendicular to a surface of the granular media;

- the excitation apparatus comprises a remote apparatus located remotely from the probe and configured for generating a time variable magnetic field, the probe comprising a unit configured for turning said time variable magnetic field into a mechanical vibration of the probe;

- the excitation apparatus comprises a vibrating apparatus located remotely from the probe and intended for vibrating the granular media.

In a second aspect, the invention relates to a method for acquiring a physical parameter in a granular media, the method comprising:

- providing an assembly, said assembly comprising:

o at least one probe comprising a housing receiving a control unit and at least one geophysical sensor connected to the control unit, and o at least one excitation apparatus,

- installing the probe at least partially in the granular media,

- generating an upward displacement of the probe with respect to the granular media using the excitation apparatus,

- receiving a signal representative of the physical parameter with the at least one geophysical sensor.

The method according to the invention may comprise one or more of the following features, taken solely or according to any potential technical combination:

- the excitation apparatus comprises:

- a vibration unit connected to the control unit and mechanically connected to a wall of the housing,

- a receiver connected to the control unit and,

- a remote emitter, located remotely from the probe,

the method comprising:

- emitting an excitation signal with the remote emitter,

- receiving the excitation signal with the receiver of the probe,

- activating the vibration unit for vibrating the probe in the granular media.

- the excitation apparatus comprises:

- a vibration unit connected to the control unit and mechanically connected to a wall of the housing,

- a receiver connected to the control unit and,

- a remote emitter, located remotely from the probe,

the method comprising:

- emitting a periodic control signal with the remote emitter at a predetermined time period,

- receiving the periodic control signal with the receiver, - activating the vibration unit for vibrating the probe in the granular media in case of loss of reception of the periodic control signal after a duration longer than the predetermined time period.

- the method further comprises deactivating the vibration unit in case of restoration of the reception of the periodic control signal by the receiver.

The invention will be better understood, based on the following description, given solely as an example, and made in reference to the following drawings, in which:

- Figure 1 is a general schematic view of an assembly according to a first embodiment of the invention,

- Figure 2 is a schematic view of a probe of the assembly of figure 1 ,

- Figure 3 is a schematic view of an assembly according to a second embodiment of the invention,

- Figure 4 is a schematic view of an assembly according to a third embodiment of the invention,

- Figure 5 is a schematic view of an assembly according to fourth embodiment of the invention, and

- Figure 6 is a schematic view of an assembly according to a fifth embodiment of the invention.

An assembly 10 for acquiring a physical parameter in a granular media 12 according to a first embodiment is schematically shown in figure 1.

The granular media 12 is for example sand. The acquisition is for example carried out in a sandy environment such as a sand desert or a sandy beach.

The granular media 12 is made of a granular material comprising a large number of distinct solid particles which are not linked by chemical bonding.

The sand of the sandy environment is for example slightly compacted. Typically, the sandy environment is made of aeolien sand deposits. In variant, the sand of the sandy area is loose.

The physical parameter is for example a ground movement, a ground velocity or a ground acceleration.

The assembly 10 according to the invention is then intended to be used for a seismic survey in the sandy environment.

The assembly 10 according to the invention comprises a plurality of probes 14 installed at least partially in the granular media 12 and an excitation apparatus 16 configured for generating an upward displacement of the probe 14 with respect to the granular media 12.

In figure 1 , the probes 14 are represented buried in the granular media 12. The probes 14 may be buried in the ground before the acquisition to reduce the noise of the recorded data, or the burying of the probes may be the consequence of the wind (sand storms), as mentioned above.

Figure 2 schematically shows a probe 14 according to the first embodiment of the assembly.

The probe 14 comprises a housing 18 receiving a control unit 20 and a geophysical sensor 22 connected to the control unit 20.

The housing 18 comprises an external wall 24 presenting a spherical shape. The external wall 24 is in contact with the granular media 12. In figure 1 , the external wall 24 is surrounded by the granular media 12.

In variant, the external wall 24 presents a cylindrical shape or an egg-shape.

The probe 14 is preferably made in a polymeric material.

In variant or in addition, the probe 14 is biodegradable and/or chemically degradable.

By“biodegradable”, it is meant that the probe 14 is made of a material which is able to be mineralized by soil microorganisms and or by air microorganisms. For example, a biodegradable material is a material in which more than 90% of the material is converted into carbon dioxide and water by the actions of microorganisms within two years, preferably within one year, more preferably within six months.

Biodegradability can be measured for example according to standard ASTM D5988- 12 whose title is“Standard test methods for determining aerobic biodegradation of plastic materials in soil”.

By“chemically degradable”, it is meant that the probe 14 is made of a material which is able to be mineralized by chemical reactions with components of the soil and/or with light, in particular with UV light. For example, a chemically degradable material is a material in which more than 90% of the material loses its structure within two years, preferably within one year, more preferably within six months.

Advantageously, the biodegradable material and/or chemically degradable material is degraded in less than within 2 years, preferably within one year, more preferably within 6 months after the contact of the probe with the ground.

Preferably, the probe 14 is made of biodegradable plastic. Biodegradable plastics are for example components which are derived from renewable raw materials.

Examples of biodegradable plastics are aliphatic polyesters, such as polyhydroxyalkanoates (PHA), like poly-3-hydroxybutyrate (PHB), polyhydroxyvalerate (PHV), and polyhydroxyhexanoate (PHH). Other examples are polylactic acid (PLA), polybutenesuccinate (PBS), or polycaprolactone (PCL). The geophysical sensor 22 comprises at least one geophone or one microelectromechanical system (MEMS) sensor.

In a variant, the geophysical sensor 22 comprises at least an accelerometer and/or thermometer.

The geophysical sensor 2 advantageously comprises three MEMS sensors.

The geophysical sensor 22 is able to sense a physical parameter, in particular a ground movement, ground velocity or a ground acceleration and to convert it into a signal which may be recorded by the control unit 20.

The control unit 20 preferably comprises a data storage unit (not shown) to store the measurements sensed by the geophysical sensor 22.

The probe further comprises a battery 26 for powering the control unit 20.

Advantageously, the probe 14 comprises an emitter (not shown) configured for emitting the recorded signal to a remote control system 27 (Figure 1 ).

In the first embodiment of the invention, advantageously, the excitation apparatus 16 comprises a vibration unit 28 connected to the control unit 20 and mechanically connected to an internal wall 30 of the housing 18.

Therefore, the vibration generated by the vibration unit 28 is mechanically transferred to at least the external wall 24 of the housing 18 of the probe 14. The vibration unit 28 is then able to vibrate the entire probe 14 in the granular media 12.

By vibrating the probe 14 in the granular media 12, the probe 14 is moved upward towards the surface of the ground by granular convection. This phenomenon is also known as granular segregation or Brazil nut effect.

The vibration unit 28 comprises for example an eccentric rotating mass actuated by the control unit 20 and powered by the battery 26.

The vibration unit 28 comprises for example an unbalanced motor.

In variant, the vibration unit 28 is an electromagnetic vibration unit.

The vibration unit 28 has a vibration frequency preferably comprised between 0.5 Hz and 100 Hz.

In the first embodiment of the example, the excitation apparatus 16 preferably further comprises a receiver 32 (figure 2) connected to the control unit 20 and configured for receiving at least a signal, and a remote emitter 34 (figure 1 ), located remotely from the probe 14, configured for emitting the signal.

In figure 1 , the remote emitter 34 is located in the remote control system 27 such as a truck. In variant, the remote emitter 34 is located in a base camp.

In another variant, the remote emitter 34 is located in a portable apparatus held by a crew on the field. Advantageously, the remote emitter 34 is configured for emitting an excitation signal.

The control unit 20 is then configured for activating the vibration unit 28 when the receiver 32 receives the excitation signal.

In this way, the recovery of the probes 14 is facilitated since the probes 14 are moved upward towards the surface of the ground and are visible for the crew for collection.

In variant or in addition, the remote emitter 34 is configured for emitting a periodic control signal at a predetermined time period.

For example, the predetermined time period is comprised between 1 minute and 10 minutes, for example 5 minutes.

The receiver 32 of the probe 14 is configured for receiving the periodic control signal. The acquisition unit 20 is configured for activating the vibration unit 28 in case of loss of reception of the periodic control signal after a duration longer than the predetermined time period.

The loss of reception of the periodic control signal is for example due to the deposit of sand by the wind above the probe 14. The additional layer of sand above the probe 14 prevents the control signal to be received by the receiver 32.

By activating the vibration unit 28, the probe 14 is moved upward to the surface and the control signal may be received again by the receiver 32.

The control unit 20 is configured for deactivating the vibration unit 28 in case of restoration of the reception of the periodic control signal by the receiver of the probe 14.

A method for acquiring a physical parameter in a granular media 12 with an assembly 10 as described above will be now described.

The method comprises providing a plurality of probes 14 and installing them at least partially in the granular media 12.

The method comprises emitting an excitation signal with the remote emitter 34, receiving the excitation signal with the receiver 32 of each probe 14, and activating the vibration unit 28 for vibrating each probe 14 in the granular media 12 generating an upward displacement of each probe 14 with respect to the granular media 12.

In addition or in variant, the method comprises emitting a periodic control signal with the remote emitter 34 at a predetermined time period.

The method then comprises receiving the periodic control signal with the receiver 32 of each probe 14 and activating the vibration unit 28 for vibrating at least one of the probes 14 in the granular media 12 in case of loss of reception of the periodic control signal after a duration longer than the predetermined time period. Therefore, the probe 14 is moved upward towards the surface of the ground and the communication with the remote emitter 34 is re-established. The receiver 32 of the probe 14 receives again the periodic control signal emitted by the remote receiver 34.

The probe 14 is then able to transfer the recorded seismic signal to the remote control system 27 for processing to obtain an image of the subsurface.

Preferably, the method comprises deactivating the vibration unit 28 of the probe 14 in case of restoration of the reception of the periodic control signal by the receiver 32. In this way, the probe 14 is maintained buried in the granular media 12 but at a depth allowing a communication between the remote emitter 34 and the receiver 32 (and/or the emitter) of the probe 14.

Advantageously, the probe 14 is configurable between an active mode wherein the receiver 32 is powered by the battery 26 and an inactive mode wherein the receiver 32 is not powered by the battery 26.

Then, the method may comprises switching the probe 14 between the active mode and the inactive mode so as to be in the active mode before the reception of the control signal and to be in the inactive mode after the reception of the control signal. This allows reducing the energy consumption of the probe 14.

In case the control signal is not received by the receiver 32, the probe 14 remains in the active mode during the activation of the vibration unit 28 until a control signal is received again by the receiver 32.

A second embodiment of the invention is shown in figure 3.

In this embodiment, the probe 14 further comprises at least one part 38 made in a ferromagnetic material. For example, the part 38 is a mass mechanically connected to an internal wall of the housing 18 of the probe 14.

In a variant, the ferromagnetic part 38 is the housing or at least a layer of the housing of the probe 14.

The excitation apparatus 16 comprises a magnetic apparatus 40 configured for generating a magnetic field 42 in the granular media 12 and for attracting the probe 14 and more particularly the ferromagnetic part 38 of the probe 14.

Therefore, the movement of the magnetic apparatus 40 above the granular media 12 generates an upward displacement of the probe 14 in the granular media 12. For example, the magnetic apparatus 40 is moved along the ground. Then the probe 14 has a substantially upward rectilinear trajectory towards the surface of the ground.

In the example of figure 3, the magnetic apparatus 40 is fixed on a mobile rover 44 remotely controlled. However, the magnetic apparatus 40 may be fixed on either mobile system. A third embodiment of the invention is shown in figure 4.

The probe 14 is similar to the probe 14 of the second embodiment.

The magnetic apparatus 40 comprises a rotating magnetic plate 46 intended to rotate with respect to the granular media 12, above the granular media 12, according to an axis substantially perpendicular to a surface of the granular media 12.

In figure 4, the plate 46 is fixed under the mobile rover 44.

By rotating the magnetic plate 46 above the probe 14, the probe 14 moves in the granular media 12 along an upward helical trajectory towards the surface of the ground.

A fourth embodiment of the invention is shown in figure 5.

The excitation apparatus 16 comprises a vibrating apparatus 48, located remotely from the probe 14 and intended for vibrating the granular media 12.

For example, the vibrating apparatus 8 is arranged on the surface of the granular media 12, and more particularly above the probe 14.

The vibrating apparatus 48 comprises for example a plate 50 configured for hitting the surface of the ground at a predetermined frequency.

The predetermined frequency is preferably comprised between 0.5 Hz and 100 Hz.

The repetitive vibrations of the ground allow the probe 14 to move upward towards the surface of the ground thanks to the granular segregation or Brazil nut effect.

A fifth embodiment of the invention is shown in figure 6.

The excitation apparatus 16 comprises a remote apparatus 52 configured for generating a time variable magnetic field 54.

The probe 14 comprises a unit 56 configured for turning said time variable magnetic field 54 into a mechanical vibration of the probe 14.

In yet another variant, the probe 14 comprises a powering apparatus 58 for powering the vibration unit 28 by electromagnetic induction by turning the time variable magnetic 54 field into electrical energy.