fig1 depicts a simplified version of a system 10 for drilling a deviated borehole through earth formations while monitoring borehole characteristics or formation properties . this system includes a drill string 12 comprising lengths of conventional drill pipe extending from the surface 14 through a plurality of earth formations 16 , 18 . the drill string 12 is located in a borehole 20 and has at one end a rotary drill bit 22 which is powered by a fluid driven or mud motor 24 . a bent sub or bent housing 26 may be provided above or below the motor 24 . the motor 24 rotates a drive shaft 28 , which is guided at its lower end by radial and thrust bearings ( not shown ) within a bearing housing 30 affixed to the housing of the mud motor . fluid , which is commonly drilling mud , is forced by mud pumps 32 at the surface down the borehole 20 to power the motor 24 . the majority of the drill string comprises lengths of metallic drill pipe , and various downhole tools 34 , such as cross - over subs , stabilizer , jars , etc ., may be included along the length of the drill string . one or more non - magnetic lengths 36 of drill string , commonly referred to as monel collars , may be provided at the lower end of the drill string 12 above the drill motor 24 . a conventional cross - over sub 38 preferably interconnects the lower end of a monel collar 36 with a by - pass or dump valve sub 40 , and the mud motor 24 is fixedly connected directly to the sub 40 . a lower bearing sub 42 is fixedly connected at the lower end of the bearing housing 30 , and contains a sealed cavity with electronics , as discussed subsequently . a rotary bit sub or bit box 44 extends from the lower bearing sub 42 , and is rotatable with the drill bit 22 . a significant advantage of the system 10 as shown in fig1 is that the entire length of the drill string 12 need not be rotated . during straight line drilling , the drill pipe , the mud motor housing , the bearing housing , and any other housings fixed to the mud motor housing are non - rotating , and the pumps 32 power the motor 24 to rotate the shaft 28 and the bit 22 . instruments sense various downhole parameters and transmit information to an mwd ( measurement - while - drilling ) tool 46 located within one of the monel collars , which then transmits the information to the surface . this information may be transmitted to the surface by pressure pulses in the drilling mud in the drill string , and is received by a near surface sensor 48 . the sensed information is then transmitted by lines 50 to a surface computer 52 which stores and processes the information for the drilling operator . if desired , information may be displayed in real time on a suitable medium , such as paper or a display screen 54 . when the drilling operator desires to form a deviation or curve in the borehole , the mud motor 24 may remain activated while the operator rotates the rotary table 56 which then rotates the entire drill string 12 . simultaneous rotation of both the drill string and activation of the mud motor 24 causes the bit 22 to drill at an offset or deviation . during this stage of drilling , the mwd system conventionally is not transmitting data to the surface , but may still sense and briefly store data within the mwd tool 46 . when the desired offset is drilled , rotation of the rotary table 56 is stopped , the drill motor 24 continues to be activated to drill the borehole at the deviated angle , and during this stage stored information may be transmitted to the surface by the mwd tool . according to the present invention , one or more sensors located very near the drill bit 22 and below the power section of the mud motor 24 provide information to a transmitter which transmits the information to the mwd tool 46 which in turn transmits the information to the surface . the significant advantage of this arrangement is that data may be sensed very near the bit 22 , rather than 20 to 100 feet up from the bit where the mwd tool is typically located . this near bit sensing allows more meaningful data to be transmitted to the surface , since the operator would like to know the characteristics of the borehole and / or the formation at a location very near the bit rather than at some location drilled hours previously . an accelerometer or inclinometer is preferably one of the near bit sensors , since information representing the inclination of the borehole closely adjacent the bit is valuable to the drilling operator . this data cannot be easily transmitted from a near bit location to the mwd tool , however , due to the presence of the intervening mud motor 24 . the necessary complexity and desirable versatility of the mud motor are not well suited to accommodate conventional data transmission lines running through the motor . it is therefore preferred that the information is transmitted from a near bit location to the mwd tool by frequency - modulated acoustic signals indicative of the sensed information . however the information my also be transmitted electromagnetically or inductively or by mud pulses , for example , and by amplitude or phase modulation or by time multiplexing rather than by frequency modulation . fig2 generally depicts in block diagram form the primary components of the system according to the present invention , and the same numeral designations will be used for components previously discussed . the lower bearing sub 42 includes a sealed cavity which houses an accelerometer 60 , a near bit acoustic transmitter 62 , a power supply 64 , and optionally one or more sensors 66 other than the accelerometer 60 . the output signal from the or each sensor is passed to a voltage - to - frequency convertor 63 which converts sensor voltage signals to frequency signals which are in turn used to modulate acoustic signals transmitted by the transmitter 62 . the signals from the transmitter 62 pass through the metal casings between the lower bearing sub 42 and an mwd receiver 70 within the monel collar 36 . the transmitted signals are acoustic signals preferably having a frequency in the range of 500 to 2000 hz . acoustic signals may be efficiently transmitted for a distance of up to 100 feet through either the drilling mud or the metal casings . alternatively , radio frequency signals of from 30 khz to 3000 mhz may be used . the mwd tool 46 includes a magnetometer or other magnetic sensor 66 , a downhole data storage device or computer 68 , an mwd acoustic receiver 70 , a power supply 72 , and an mwd mud pulse transmitter 74 . although it is generally preferred that the borehole or formation characteristics be sensed at a location below the drill motor 24 , the magnetometer must be magnetically isolated from the metal housings for reasonable accuracy and reliability , and accordingly it is housed within the monel collar 36 . if desired , other sensors , such as backup sensors , could also be provided within the monel collar 36 , although preferably sensors other than the magnetic sensor are located at the near bit location . in addition to the inclinometer or accelerator 60 , near bit sensors provided within the sub 42 may include a weight - on - bit sensor , a torque sensor , a resistivity sensor , a neutron porosity sensor , a formation density sensor , a gamma ray count sensor , and a temperature sensor . data from each of these sensors may thus be transmitted by the transmitter 62 to the mwd receiver 70 . the computer 68 includes both temporary data storage and data processing capabilities . in particular , information from various sensors may be encoded for each sensor and arranged by the computer so that like signals will be transmitted to the surface , with the signals from each sensor being coded for a particular sensor . porosity signals , magnetometer signals , resistivity signals , inclination signals and temperature signals may thus be intermittently transmitted to the surface by the mwd transmitter 74 . the receiver 70 , computer 68 , transmitter 74 and any sensors within the monel collar are all powered by the power supply 72 . fig3 shows the lower bearing sub 42 at the lower end of the bearing housing 30 which is in turn secured to the end of the bent sub or bent housing 26 . the sub 42 incorporates a sealed annular cavity 76 for the near bit sensing components shown schematically in fig2 within the sub 42 . in non - illustrated variants of the invention the sub 42 may be part of the assembly consisting of the mud motor 24 or the bearing housing 30 , and optionally may also include the bent sub or housing 26 , and the sealed cavity may be formed by the sub 42 or by the housing for either the mud motor 24 , the sub 26 or the housing 30 . alternatively the cavity may be formed in the drill bit itself . the lower bearing sub 42 includes an integral recessed lower body 80 to define the cavity 76 , and an outer sleeve 82 which is threadably connected to the body 80 , with a fluid - tight seal being formed by o - rings 84 and 86 between radially outer portions of the body 80 and the sleeve 82 . a wear sleeve 92 and a radial bearing 88 are positioned within the sub 42 . the inner cylindrical surface of the radial bearing 88 is slightly less than the inner diameter of body 80 , so that a sleeve extension 90 of a lower spacer sleeve normally engages the radial bearing 88 but not the body 80 . the spacer sleeve and thus the extension 90 are attached to a mandrel 94 , which is rotated by the drive shaft 28 , so that the sleeve extension 90 and the mandrel 94 rotate with respect to the body 80 . a mandrel ring 96 is attached to the mandrel 94 to secure the lower end of the sleeve extension 90 in place . the mandrel 94 defines a cylindrical full bore 98 for passing the drilling fluid to the bit , and the bit box 44 may be threadably secured directly to the lower end of the mandrel 94 . the sealed cavity 76 houses the acoustic transmitter 62 , the accelerometer 60 for measuring the component ( gz ) of the earth &# 39 ; s gravitational field in the axial direction of the drill bit , the voltage - to - frequency convertor 63 and the power supply 64 which may consist of a lithium battery pack or generator assembly . any number of additional sensors represented by 66 may be provided within the sealed cavity to monitor near bit information . if desired , a small computer may also be provided within the cavity 76 to provide temporary data storage functions . the computer may include timing programs or signal conditioning circuitry to regulate the timing for transmitting frequency modulated acoustic signals for each of the sensors from the transmitter 62 to the receiver 70 . also , a turbine or eddy current generator 65 may be provided for generating electrical power to recharge the battery pack 64 or to directly power the sensors , computer and transmitter within the cavity 76 . the generator 65 is stationary with respect to the adjoining rotary mandrel 94 , and accordingly my be powered by the mandrel driven by the motor 24 . referring to fig4 the components housed within the sealed cavity 76 are located within a split cylindrical potted mould 100 , shown in fig4 comprising a battery mould part 101 and an electronics mould part 102 for the other components . the battery mould part 101 has three axially extending arcuate chambers 103 , each of which contains a respective moulded silicone rubber sleeve 104 for accommodating four pairs of lithium batteries side - by - side . the battery mould part 101 also includes wiring ( not shown ) connecting the batteries to an electrical connector 105 for engaging a complementary connector ( not shown ) on the electronics mould part 102 . the electronics mould part 102 has an axial chamber 106 for the transmitter 62 , three recesses 107 for circuit boards 108 of control circuitry and an axial chamber 109 for the accelerometer 60 . although not visible in fig4 the electronics mould part 102 also has a recess for a tensioning device which tensions a retaining strap for extending around the two mould parts 101 and 102 to retain the mould parts in position within the cavity 76 . the control circuitry includes an analogue control circuit for the accelerometer 60 , a signal conditioning circuit for encoding the sensor data for transmission , and a timing circuit for enabling the transmitter to be powered on after a preset delay . in addition circuitry may be provided for actuating the transmitter only after drilling has stopped , either in response to an acoustic pickup which senses that drilling noise has stopped or in response to an acoustic signal from the mwd receiver 70 sensed by a piezoelectric receiving device . in addition the battery mould part 101 has detachable upper and lower covers ( not shown ). referring to fig5 which shows a section through the electronics mould part 102 taken along the line v -- v in fig4 the acoustic transmitter 62 comprises two coaxial cylindrical pole pieces 110 and 111 separated by an annular air gap 112 and interconnected by an axial rod ( not shown ) made of magnetostrictive material . the axial rod is surrounded by a cylindrical coil within the pole piece 111 , and the supply of a suitable input signal to the coil results in physical deformation of the rod in such a manner as to produce an acoustic output signal . the air gap 112 is provided to allow the rod to extend and contract without constraint , and a prestress system including a compression string 113 surrounding a stud 114 serves to compress the pole pieces 110 and 111 in the axial direction . those skilled in the art should now appreciate the numerous advantages of the system according to the present invention . a fast , accurate , and low cost technique is provided for reliably obtaining and transmitting valuable near bit information past the drilling motor and to the surface . in particular , well bore inclination may be monitored at a near bit position , although well bore direction may be reliably sensed and transmitted to the surface from a position above the drill motor . complex and unreliable hard - wiring techniques are not required to pass the information by the drill motor . although reliable near bit information is obtained , the sensors are not normally rotated during ongoing drilling operations , so that the sensors and electrical components within the sealed cavity 76 are not subject to centrifugal forces caused by drill bit rotation in the 50 to 6000 rpm range . also , if required , data may be transmitted to the surface during the drilling mode , thereby saving valuable drilling time . moreover , the sub 42 is substantially isolated from the high vibrational forces acting on the drill bit due to the various bearing assemblies within the bearing housing 30 . the angular or orientational position of the sensors within the sealed cavity 76 is fixed , and thus the position of any sensor with respect to the sub 42 and thus the drill string 12 may be determined and recorded .