& gt ;& gt ; embodiment according to the first detection principle with sound assistance , validation by simulation of digging a virtual hole and display of the treasure fig1 shows a chest ( 1 ) containing an electronic circuit , battery powered , connected to a coil ( 3 ). the chest is buried under the ground ( 2 ). the electronic circuit connected to the coil ( 3 ) generates an alternating current therein , which results in creating a magnetic field ( 4 ) that develops above the surface of the ground ( 2 ). this magnetic field is modulated and encoded by the electronic circuit connected to the coil ( 3 ). the treasure simulator , although invisible , can be referenced and identified owing to the use of a special detector according to the invention . in reference to fig2 , the chests of the pseudo - treasures ( 1 ) are positioned under the ground ( 2 ) distributed over a surface called playing area ( 5 ). each pseudo - treasure produces a field ( 4 ) on a reduced surface of the playing area . this field is sensed by a detector ( 6 ) according to the invention . the player ( 7 ), through a scanning motion and movement of the coil of the detector ( 18 ), looks for a treasure hidden under the playing area ( 5 ). the detector ( 6 ) emits a sound signal in the headset ( 8 ) and light animations of the spiral ( 9 ) that surrounds the support tube ( 10 ) when it is in the presence of the magnetic field of the pseudo - treasure whereof the code corresponds to the treasure that the player must find . the detector broadcasts a sound message to announce to the player that he has detected a treasure , then invites the player to move toward the barrels ( 11 and 12 ) to dig a virtual hole . in reference to fig3 , which shows the simulation of digging in three stages , in the first stage one can see the presence of two barrels shown in cross - section . the first barrel ( 11 ) contains balls ( 13 ) that rest on a conical surface ( 14 ) fixed on the bottom ( 15 ) and the side of the barrel . the bottom is equipped with a magnetic transmitter ( 16 ) identical to the transmitters of the pseudo - treasures , but with a very low power . the second barrel ( 12 ) is similar to the first but , initially , does not contain balls . the player , to simulate the digging of a hole , must manually transfer all of the balls , including the last one ( 17 ), into the second barrel . in the second stage , the barrel ( 11 ) being completely empty , the player can submerge his detector ( 6 ) in the barrel and put the coil of the detector ( 18 ) in contact with the transmitter of the bottom ( 16 ). the virtual hole having been dug , the detector triggers a sound message that invites the player to plug them back up by removing his detector ( 6 ) from the first barrel ( 11 ), then transferring all of the balls ( 13 ) from the second barrel ( 12 ) to the first barrel . in the third stage , all of the balls have been returned into the first barrel ( 11 ), which allows the player to submerge his detector ( 6 ) in the second barrel ( 12 ) until it comes into contact with the transmitter of the second barrel ( 19 ). the detector informs the player , by a sound message , about the nature of the virtual treasure he has just discovered . the detector transmits a code to the game management computer ( 24 — fig4 ) via the infrared diode ( 98 — fig6 ) incorporated into the center of the coil ( 18 ). an infrared phototransistor integrated into the transmitter of the second barrel ( 19 ) is connected to an input of the management computer which , using appropriate software , is able to display an image of the treasure on the video projector screen ( 25 — fig4 ). at this stage , the detector broadcasts a quiz in the headset to which the player will respond by pressing on push buttons . a correct response allows the player to find another treasure on the playing area more quickly . fig4 shows the general organization of the game . players ( 7 , 20 , 21 ) are present on the playing area ( 5 ). a welcome counter ( 22 ) receives the game management computer ( 24 ). this computer is connected to a video screen ( 25 ) or a large format tv monitor , the second barrel ( 12 ) or , in prize distribution mode , a box ( 23 ). these connections can be wired or a hf connection . at the beginning of the game , the organizer , using the game management computer , introduces the player &# 39 ; s goal table into the memory of the detector via a serial connection ; as a result , the player must find his pseudo - treasures in several steps in the playing area ( 5 ) and makes , in each step , his pseudo - holes with the barrels ( 11 and 12 ) in a predetermined time to win the game or a prize ( examples of game rules ). the game management computer includes , in its hard disk , multiple game levels and themes ; the operator can therefore adapt the detector to the player &# 39 ; s age and the themes of the game . outside the playing area ( 5 ) can be a semi - hard ground surface such as dirt , lawn or a loose type such as sand or gravel ; in these cases , the pseudo - treasures are buried . inside or when the ground is hard , the playing area can be a carpet or a parquet floor incorporating the transmitters in miniature form . fig5 shows that the oscillator of the microcontroller clock ( 26 ) of the pseudo - treasure is controlled by a quartz ( 27 ) and two capacitances ( 28 and 29 ) to ensure good stability . the microcontroller ( 26 ) generates a square signal modulated by a shift of two low frequencies . these shifts of two very close frequencies ( for example 24 and 26 khz ) correspond to a code according to the so - called fsk mode . this signal is transmitted via the resistance ( 30 ) but limited to 0 . 7 v by the diode ( 31 ). the capacitance ( 32 ) and the resistance ( 33 ) present the signal to the input of an integrating circuit made up of the operational amplifier ( 34 ) of the resistance ( 35 ) and the capacitance ( 36 ). this integrating circuit is calculated so as to convert the square signal into a triangular signal that is presented via the resistance ( 37 ) to the input of a filter formed by the amplifier ( 38 ), the resistance ( 39 ) and the capacitance ( 40 ). at the output of this filter the signal is sinusoidal , the resistance ( 41 ) transmits it to the amplifier ( 42 ) whereof the gain fixed by the resistance ( 43 ) allows the signal to rise to 3 v . the resistance ( 44 ) leads the signal toward the unitary gain amplifier ( 45 ), the output of which via the resistance ( 46 ) injects , into the tuned circuit made up of the transmission coil ( 47 ) and the capacitance ( 48 ), the frequency - modulated encoded signal . in the case where the ground has a low magnetic impedance , the transmission power can be increased by the placement in parallel owing to the switch ( 49 ) of the resistance ( 50 ) with the resistance ( 46 ). a divider bridge made up of resistances ( 51 and 52 ) has a voltage filtered by the capacitance ( 53 ) at each of the inputs + the four amplifiers . the treasure simulator comprises a butzer ( 54 ) connected on an outlet of the microcontroller via a resistance ( 55 ) which informs the operator by a beep during powering on by the switch ( 56 ) of the powering on . a brown - out detection circuit , made up of a fixed voltage source , formed by the zoner ( 57 ) and the resistance ( 58 ) and a threshold detection done with the transistor ( 59 ) and the resistance ( 60 ) aims , once the reserve level of the battery ( 61 ) is reached , to change the level of an input of the microcontroller . the latter causes a series of beeps in the butzer ( this function makes it possible to find the treasure simulator easily when it is buried ) and the transmission of a specific code called low battery to the detector . through this principle , the operator is informed at the end of each game of the level of the batteries of the pseudo - treasures that have been detected by the player . when the pseudo - treasures are permanently buried ( fixed playing area ) or included in a carpet or a parquet floor , the batteries are replaced by a low - voltage power cable shared by all of the pseudo - treasures . fig6 shows the presence of a detecting coil ( 18 ) situated at the end of the detector . this coil is designed to receive , by mutual induction , the magnetic field created by the coil of the pseudo - treasures . it also forms , with the capacitor ( 62 ), a band - pass filter that eliminates the frequencies other than the emission frequencies of the pseudo - treasures . the resistance ( 63 ) and the two head - to - tail diodes ( 64 ) constitute a safety by limiting the voltage against electrical shocks . the signal is presented via the capacitance ( 65 ) to a first non - linear specific amplifier stage ( 66 ). the signal at the capacitance ( 65 ) varies in very significant proportions as a function of the scanning motion made by the player , in fact the coupling factor varies as a function of the cube of the detection distance . the amplifier stage must have a high input impedance and a gain inversely proportional to the signal present at the input without reaction delay . the solution to this specific need is provided by the presence of the head - to - tail diodes ( 67 ) mounted in parallel with the reaction resistance ( 68 ). the polarization at half of the power voltage of the input + the amplifier ( 66 ) is ensured , via the resistance ( 69 ) by the resistance divider bridge ( 70 and 71 ). the capacitance ( 72 ) ensures the filtering and virtual reunion of the alternating ground . when the current caused in the coil ( 18 ) is weak ( detection distance up to 80 cm for a transmission power lower than 1 mw ), the diodes do not conduct , the gain of the amplifier ( 66 ) is very significant . when the player presents the coil at the vertical of a pseudo - treasure , the field and the current caused are maximum , the diodes ( 67 and 68 ) conduct and therefore the amplifier has a low gain . the signal is transmitted by the connecting capacitance ( 73 ) to a polarization bridge made up of resistances ( 74 and 75 ) then to the minus input of the internal comparing element of the microcontroller ( 79 ). the plus input of the comparing element is connected to the divider bridge formed by the resistances ( 80 and 81 ), the capacitor ( 82 ) ensures the filtering . this mounting compares the evolution of the signal with a level corresponding to the virtual mass , which creates a level change of the output of the comparing element internal to each sinusoid . the fm demodulation of the code emitted is ensured by this circuit and suitable software . the stabilization of the clock of the microcontroller ( 79 ) is ensured by the quartz ( 83 ) and the capacitances ( 84 and 85 ). the microcontroller ( 79 ) is connected to a sound synthesis circuit ( 86 ) comprising a removable memory ( 87 ). the sound synthesis output is connected to an audio amplifier ( 88 ) whereof the gain can be adjusted owing to a potentiometer ( 89 ), this amplifier is connected to a headset ( 90 ). a speaker ( 91 ) makes it possible to broadcast sounds synthesized by the microcontroller ( 79 ). push buttons ( 92 and 93 ) through which the player can respond to the quizzes are connected to the microcontroller ( 79 ). the powering on of the detector is done by a switch ( 95 ); the power supply is ensured by a battery ( 94 ). in the case where the player has placed the coil of his detector near one of the pseudo - treasures ; if the code is the same as the stored goal , the microcontroller transmits an instruction to the sound synthesis ( 86 ) that broadcasts a sound signal evoking a radar echo whereof the frequency varies as a function of the received magnetic field level . this sound signal is broadcast in the headset ( 8 ) or in the speaker ( directly from the microcontroller ). on the other hand during the approach , a radar screen visual simulator ( 96 ) made up of led diodes has emitted concentric slivers as a function of the level of the received magnetic field . the level increase also causes an increase in the frequency of the flashes emitted by the light spiral ( 9 — fig2 ). the level is detected by the diode ( 76 ), which rectifies the signal coming from the amplifier ( 66 ), the capacitance ( 77 ) and the resistance ( 78 ) form a time constant that determines the reaction speed of the detector to the variations of the field . this variable continuous signal is then presented at the input of the analog - digital converter inside the microcontroller by software : this data frequency modulates the sound and light animations previously described . the detector then broadcasts a message to a player inviting him to dig a virtual hole under the conditions described by the associated text in fig3 , the player places his detector at the bottom of the first barrel . the detector receives the code of the first barrel emitted by the transmitter ( 16 ), after concordance verification the microcontroller ( 79 ) triggers the synthesis ( 86 ), which broadcasts a message inviting the player to return the balls to their initial place ; when the player introduces his detector into the bottom of the second barrel , the microcontroller ( 79 ) senses the code of the second barrel and emits a digital signal encoded by frequency modulation in the resistance ( 97 ) connected to the infrared diode ( 98 ). this signal will be picked up by the phototransistor situated at the bottom of the second barrel , then transmitted to the management computer . the first goal being found , the microcontroller ( 79 ) stores this result and transmits instructions to the synthesis ( 86 ) or the display ( 99 ) so as to broadcast a quiz . the player answers it by actuating the yes button ( 91 ) or the no button ( 92 ). if the response correct , the microcontroller broadcasts a clue allowing the player to find his next goal . lastly , the microcontroller ( 79 ) signals the end of the game to the player , which can be due to exceeding the allowed time or successfully finding all of the goals of the game . the principle of the system according to the invention and all of the devices making up that system allows several players ( 20 , 21 , 8 — fig2 ) to play on the same playing area ( 5 — fig2 ) with a limited number of pseudo - treasures ( 1 fig2 ) which remain in place from game to game ; the program of the steps of the game and the quizzes have been introduced into memory by the serial connection ( 93 ) at the beginning of the game . fig1 shows a time schedule managed by the software , according to one of the principles of the invention , installed in the management computer , a configuration is observed described as an example comprising 20 pseudo - treasures . one can see four - column tables ( 121 ), each line represents the four goals of each player , the columns represent the maximum times ( 3 minutes ) for each goal . the origin of the tables is shifted to the right as a function of the arrival of the players from the origin of the axis of the times ( 123 ). the twenty treasure numbers ( recorded in the boxes ) are randomly distributed for the first 5 players . in the stage shown by a broken line ( 122 ), apparently no more available pseudo - treasures remain , no player having returned his detector . the game management software calculates the pseudo - treasures which , although part of the goals of players present in the playing area , are already available since the maximum times to discover them have elapsed . as a result , pseudo - treasure no . 3 can be allocated to player no . 6 while player no . 1 is present on the playing area . the tables of goals for players nos . 6 , 7 and 8 are randomly formed with the available pseudo - treasures and the treasure numbers that soon will be in minute 11 , player no . 8 is assigned treasure no . 7 , which is effectively available , but also no . 20 that player no . 1 is looking for at that time . this possibility exists since player no . 8 will look for that treasure 9 minutes later ; player no . 1 at that stage will have finished his game . this optimization device is intended to absorb the peak flows of players . during normal operation , the game must have a much larger number of pseudo - treasures than the number of players present on the playing area so as to preserve the random nature of the games . in case of extended saturation , the software would propose to the operator to organize games including three goals , or 9 minutes . called zone preselection , usable outside buildings and over a large playing area . in reference to fig7 , the microcontroller receives , from a gps circuit ( 100 ) or a position detection circuit , the spatial coordinates of the detector at any time . the goal table introduced into the memory of the microcontroller ( 79 ) is made up of perimeter coordinates of each of the small zones in which the pseudo - treasures to be found are situated and the maximum times granted to perform the detection . the pseudo - treasures are passive ; they do not emit a code , but can be located owing to the detector given to each player . the microcontroller triggers the treasure found process when the detector is situated on a position that is within the perimeter of the fixed goal zone and the sensing device has detected the pseudo - treasure by field absorption . according to this detection principle , the digital analog converter of the microcontroller ( 97 ) produces a summary low - frequency sinusoidal current . this current passes through the active filter made up of the resistances ( 101 and 102 ), the capacitance ( 103 ) and the operational amplifier ( 110 ). the idle point of this amplifier is set by the dividing bridge made up of the resistances ( 80 and 82 ) filtered by the capacitance ( 82 ). the current passes through the resistance ( 104 ) and the unitary gain amplifier ( 106 ) to be injected via the resistance ( 107 ) into the oscillating circuit made up of the coil ( 108 ) and the capacitance ( 107 ). the oscillation level is taken between the resistance ( 108 ) and the capacitance ( 107 ) to be presented at the input of the analog digital converter of the microcontroller ( 79 ). the other elements of the assembly of fig7 have the same function as those of fig6 . according to this principle , the pseudo - treasure is a tuned circuit ( inductance capacitor ) on the emission frequency of the detector , or any other absorbent device , for example a thomson ring . in the presence of a pseudo - treasure , the voltage measured by the nd converter will decrease ; the zone corresponding to the goal , the found treasure process is triggered by the microcontroller . the pseudo - detectors given to the players comprising a stable oscillator that powers an infrared led diode produce an infrared radiation modulated on a frequency specific to them . fig9 shows that the infrared photo - detectors ( 115 ) are integrated into a carpet ( 118 ) serving as playing area . the photo - detectors are protected by an infrared - transparent but opaque plastic sheet ( 117 ) in the visible range so as to conceal the pseudo - treasures from public view . in reference to fig8 , a matrix ( cabled in x y ) of sensors ( 115 ) is connected to a detection interface . this interface comprises two ports ( 113 , 114 ) that receive from the sensors a digital signal whereof the frequency varies as a function of the pseudo - detector producing the signal . in each game , only several inputs of the ports are active , thereby determining several pseudo - treasures on the playing area . the microcontroller ( 109 ) is equipped with a precise clock made up of a quartz ( 111 ) and two capacitances ( 110 and 112 ); it converts , owing to that clock and software that performs a comparison with the frequency table stored in memory , the received player code frequency . this code as well as the pseudo - treasure number obtained by selecting inputs are communicated via a serial connection ( 116 ) to a game management computer that compares the code picked up with the player &# 39 ; s goal table . this matrix - based principle is adapted to small indoor economical playing areas . one alternative can be produced by replacing the led diodes and the photo - detectors with coils producing an electromagnetic field . the system according to the invention is intended to be exploited in recreational locations ( beaches , amusement parks , party or game rooms , etc .) or used as an advertising promotional tool on the sales sites . it may be the basis for a televised game , a website or a radio show .