before describing in detail the preferred apparatus and methodology of the invention , the several of the basic concepts employed in the invention will be presented as a foundation for more detailed disclosure . attention is first directed to fig1 a of the drawings which is a simplified view showing a well during drilling and a well which requires a survey . to provide a context for the method and apparatus of the present disclosure , fig1 a shows a well borehole 10 which extends into the earth &# 39 ; s surface and which has some measure of deviation . the amount of deviation is significant in many instances . to provide a suggested minimum , fig1 a will be described assuming that the well includes an upper portion which is more or less vertical and a central or lower portion which is inclined at an angle in excess of about 15 °. typically , the well is surveyed at some time during drilling , and especially when drilling a deviated well . surveys typically are not required when the well is primarily vertical or when the well is relatively shallow . sometimes , the type of survey made by the present system is not conducted in vertical wells . this type of survey carries a premium charge in comparison with lesser techniques preferred in the survey of vertical wells . indeed , it may be sufficient merely to drill the well completely without this type of survey equipment should the well be totally vertical and relatively shallow . the present invention is best applied to deeper wells and those which have deviated portions . typically , this well is surveyed before it has been cased from top to bottom . there may be a portion of casing equipment at the top part . again , the casing may be present only through a few hundred or a few thousand feet of depth . in many instances , the well may be simply open hole . whatever the circumstances , the present disclosure sets forth the well at a preliminary stage . the well of this disclosure is surveyed by providing a wireline supported instrument probe 20 . a drum 12 spools and deploys the wireline cable 14 on the drum thereby conveying the probe 20 along the borehole 10 . it is directed into the well through a pulley 16 at the surface , which is often referred to as a “ measure ” or “ sheave ” wheel . this pulley also serves as a guide wheel for directing the wireline cable 14 into the wellbore 10 , and also serves as an input device for depth measuring equipment ( dme ) 18 which measures the length of wireline 14 that extends into the wellbore 10 . at the bottom of the wireline 14 , the survey instrument probe 20 of the present disclosure is supported . the survey instrument 20 comprises an elongate cylindrical shell or housing . the equipment to be discussed below is supported on the interior . the equipment shown in fig1 a additionally includes a clock 22 which provides data for a time based recorder 24 . that forms a printed record 26 of measured and computed wellbore survey data . the survey record 26 starts at t o and runs to t f . the time t o therefore represents the beginning instant of the survey and t f represents the end of the survey . the record 26 is a recording of survey data as a function of time , or can alternately be converted as a function of the depth of the survey instrument probe 20 along the borehole 10 , where depth is measured by the dme 18 by sensing the length of wireline 14 deployed within the borehole 10 . fig1 a additionally shows a reference system which is tied to the instrument . the z axis coincides with the elongate axis 21 of the housing 20 and also coincides with the axis of the borehole 10 . at the surface , the x and y axes coincide with a horizontal plane which is transverse to the well borehole 10 . as will be understood , this reference system moves with the instrument . when the instrument 20 moves into the deviated portion , that repositions the reference system . in addition , fig1 a shows the gravity factor which is represented by g . to the left and right of the probe instrument package 20 , the x and y axes define the plane which is horizontal at the surface but which is otherwise tilted depending on the inclination of the survey instrument 20 . by viewing the instrument along the x axis as shown in fig1 b , the y axis is shown at an inclined angle above the horizontal as illustrated in fig1 c . as mentioned previously , two measurement principles , the gyrocompassing technique and the continuous survey mode , are employed to calculate wellbore trajectory as a function of depth . these measurement principles , and their application to the desired measurement , will be briefly summarized . the gyrocompassing survey technique is employed to survey near vertical wellbore sections , and to measure the initial heading reference prior to switching to the continuous mode . during the gyrocompassing procedure , the probe 20 is lowered into the wellbore 10 by means of the electric wireline 14 to measure the earth &# 39 ; s gravity field and the earth &# 39 ; s rate of rotation while the probe is held stationary at predetermined depths . x and y accelerometers , denoted as a pair by the numeral 32 , measure the gravity field , g , with respect to the axis 21 of the instrument probe 20 as shown in the schematic , three dimensional prospective fig2 . the measured quantities are the orthogonal vectors a x and a y shown in fig2 . the azimuthal orientation of the probe 20 within the borehole 10 defines the “ highside tool face ”, see the accelerometer vectors in the plane at right angles to the housing axis in fig6 and 10 . an accelerometer measures acceleration ( in this particular invention the earth &# 39 ; s gravity field ). the vector combination of the two accelerometers enables measurement of the instrument axis roll or the tool face angle of the instrument . this is performed by determining the ratio of the x - axis accelerometer output over the y - axis accelerometer output . in addition , the accelerometer outputs enable one to determine how far the instrument is deviated from vertical . in other words , the accelerometers define the inclination of the wellbore at a measured depth . in order to do so , the x - axis accelerometer output and the y - axis accelerometer output are projected onto the highside of the crossborehole plane of the instrument . the angle between the projected highside gravity component and the earth &# 39 ; s gravity field define the inclination of the wellbore at that particular measured depth . see fig6 and 10 for visual clarification . this allows the computation of the inclination of the probe 20 , therefore the inclination of the borehole 10 at the position of the probe along the well path 10 ′, to be measured . the computation is performed by means of mathematical projection of the gravity field vector g into the accelerometer sensitive axis plane defined by a x and a y . it is apparent that the accelerometer readings alone are not sufficient to map the path 10 ′ of the borehole in three - dimensional space , since the heading azimuth of the borehole , shown in fig2 is not known . this is provided by the gyro readings as described in the following paragraph . the rate gyro sensor 30 measures the earth &# 39 ; s rate of rotation , defined by the vector ω , identified by the numeral 61 in fig3 . since the earth rotates at a fixed speed and these measurements are made at a given latitude 63 . the vertical and horizontal components of the earth rate vector components ω , defined as e h and e v , respectively , can be derived as shown in fig3 . note that the component e v forms an angle φ , with the plane 65 defining the earth &# 39 ; s equator , therefore defining the latitude of the well borehole . the components e h and e v can then be projected into the sensitive gyro axis plane , ( g y , g x ) where g y and g x are the angular rate outputs of the gyro 30 , and where the horizontal earth rate component e h references true north as shown in fig4 . the rate gyro , therefore , provides an reading of the azimuth 67 of the well path 10 ′, referenced to a fixed direction such as true north . by combining the output of the gyro sensitive axes ( g y , g x ) and the accelerometer outputs a x , a y , the well bore direction , inclination , and tool face highside can be determined . depth is incorporated from the amount of wireline 10 deployed from the drum 12 to lower the probe 20 within the borehole 10 . combining a series of survey stations downhole through a calculation method such as minimum curvature yields wellbore trajectory path 10 ′. the continuous survey mode is based on measuring relative instrument rotations while the probe 20 is continuously traversing through the borehole 10 . after taking a stationary reference heading measurement in the gyrocompassing mode , new modeling procedures allow computation of probe azimuth and inclination changes , da / dt and di / dt , respectively , about the highside ( hs ) and highside right ( hsr ) directions , where the hsr direction is at right angles with respect to the hs direction . this is accomplished by mathematically projecting da / dt and di / dt into the gyro sensitive axis plane ( gy , gx ), as shown in fig5 . in order to calculate the actual wellbore path , the rate of rotation about hs and hsr are integrated over time , yielding wellbore heading and inclination changes from the previously described reference procedure . in conjunction with depth , which is derived by continuously monitoring the amount of wireline 14 deployed , the wellbore trajectory 10 ′ is generated . recall that the system is operated in the gyrocompassing mode with the survey probe stationary in order to obtain a reference azimuth a and a reference inclination i . in the subsequent continuous mode of operation , the survey probe is conveyed along the borehole , the variation of inclination and azimuth , with respect to the reference inclination and azimuth is measured , and the path or trajectory of the wellbore in three - dimensional space is computed from these measured rates of change . the operation , data processing , and results obtained in both modes of operation will be disclosed in detail . as shown in fig1 a of the drawings , the portion of the well which is substantially straight does not require the expensive type survey which is conducted by the present disclosure . accordingly , the survey instrument 20 need not be run in that portion . it is better to survey that portion of the well with the gyro compass system only . it is also better to run the survey in the highly inclined portion . fig1 a shows the instrument probe 20 in the radically inclined portion of the well . the survey instrument of the present disclosure is especially effective at inclined angles in excess of about 20 ° or perhaps even 15 ° up to above 90 °. in a vertical well , the accelerometers ( at right angles to gravity ) do not provide an output data . inclination is needed to prompt accelerometer readings . a maximum inclination is not defined . in other words , at that juncture the instrument probe 20 is almost laying in a horizontal wellbore 10 . moreover , the survey instrument and procedure of the present disclosure is best carried out while collecting four data streams from the survey instruments in the survey probe 20 . the gyro sensor 30 provides a rate gyro signal . as the z axis of the gyro is forced from coincidence with the vertical , angular rates are generated . these are rates normally expressed in angular rotation per unit time such as degrees / min . there are two components of the angular rotation rate . the axis of the gyro 30 will be tilted with angular tilt being measured as it is rotated from a true vertical position . imposing a reference system on the gyro in the perfect upright position , one component of information is the angular rate or g x and a similar angular deflection is g y . the two measurements are both needed because it would be a rare circumstance in which deflection were totally in only the x or y dimensions . therefore the output of the gyro instrument 30 within the survey probe 20 is g x and g y . as will be understood , the gravity vector is represented by the vector g . the accelerometers 32 form the output signals a x and a y . there is no need to deploy an accelerometer along the z axis and hence there is no data a z . if z axis data is needed , it can be alternately obtained from the wireline movement , and that information as needed is available from the dme data . in fig6 and 7 jointly , the gravity vector g again is shown . fig6 shows in abbreviated fashion the case or housing 20 . it has imposed on it the designation at 34 indicating the highside of the tool face . this is the uppermost point on the housing 20 in a transverse plane with respect to the tool axis . the point 34 is located in a plane 36 at right angles to the hole axis and spin axis 21 of the survey probe 20 . this plane is defined in the x and y dimensions . in fig6 it is shown from the side , but at an angle dependent on the angle of deviation of the well . this permits rotation of the plane 36 to the horizontal as shown in the full line representation in fig6 and which is projected into fig7 by the dotted line representation . the highside point 34 is rotated into the horizontal plane shown in fig7 . recall that the gyro 30 has two axes which are maintained in alignment with the x and y accelerometer axes . recall also that horizontal earth rate vector e h can be readily resolved into vector components . this is shown in part in fig7 where the vector 40 is resolved into x and y components . this is the vector that is indicative of true north and includes the vectoral components resolved in fig7 . when that rotation is made , thereby resulting in the projection of the true north vector in the horizontal plane as shown in fig7 the true north vector can then be seen . the present system forms data which yields the true north measurement which is then converted into the azimuth as shown in fig7 . this is the previously discussed reference azimuth a obtained with the system operating in as a station measurement the gyrocompassing mode . operation should be considered now . if the probe 20 is suspended in a vertical wellbore , the accelerometer outputs which are a x and a y are insensitive to gravity . when the well is deviated as shown in fig1 a by an amount sufficiently large to define two components , it is possible to represent at least the x and y components of the gravity vector g so that vector components can be resolved in the x - y plane . these are represented as a x and a y which are added as vector components to obtain two measures of the gravity vector . the vector addition of components a x and a y yields the direction of the highside ( hs ) of the instrument in the borehole 10 at the position of the probe 20 . mathematical projection of the output of the x - axis accelerometer and the output of the y - axis accelerometer onto the highside direction provides the projected gravity component sensed by the instrument . the angle between the projected gravity component sensed by the instrument and the gravity direction equals the wellbore deviation angle when the instrument is stationary . the multiple mode of operation is triggered in many ways , for example , by a switch , or by arbitrary depth selection or by computer operation . if several wells are drilled straight below a platform for 1 , 500 feet and then deviated to reach an underwater field , the first 1 , 500 feet of hole need not be surveyed . the continuous mode is switched on after 1 , 500 feet . restated , no survey is needed for 1 , 500 feet and the time to is started then . this is implemented by turning on the power supply and data processor at to after 1 , 500 feet . a switch in the data processor is sufficient . once the reference azimuth and reference inclination values , a and i , have been measured with the probe 20 stationary , the continuous mode of operation is initiated . the gyro 30 is locked using a locking apparatus described in the following section . the computation of inclination i c and azimuth a c values in the continuous mode , with respect to corresponding reference values i and a measured in the stationary , gyrocompassing mode , is presented in block diagram form in fig8 . the accelerometer outputs a x and a y , represented by boxes 208 and 212 , are used to form the ratio a x / a y at the step represented by step 222 . the outputs g x and g y , represented by the boxes 200 and 204 , respectively , are combined with this ratio at step 222 to correct the ratio for any non gravity acceleration effects . the computation at step 222 yields the rate of roll over the hsr direction with respect to a reference rate of roll . this quantity is integrated over time , measured from a previously mentioned reference time to , which represents the initiation of the continuous mode operation , and combined with g x and g y at step 224 to yield a relative borehole inclination . this relative borehole inclination , when combined with the reference borehole inclination 214 stored in a memory device 220 , yields the desired borehole inclination i c with the system operating in the continuous mode . the i c output is represented at 230 . still referring to fig8 the relative borehole inclination , g x and g y , and a x / a y , are combined and integrated over time , measured from to at step 226 . this yields a continuous relative azimuth value measured with respect to a , the reference azimuth 216 stored within the memory 220 . the relative azimuth is combined with the reference azimuth a at step 226 to yield the desired azimuth reading a c , represented at 240 , which in with the azimuth of the borehole computed with the survey system operating in the continuous mode of operation . as discussed previously , i c and a c are combined to yield a map of the borehole in three - dimensional space . all computations are preferably performed at the surface using a central processing unit defined in the following discussion of the system apparatus . to summarize , a c and i c are determined mathematically by integrating , over time , measured rates of change of inclination and azimuth with respect to measured , reference azimuth and inclination values . this approach greatly simplifies the downhole equipment required to obtain and accurate and precise map of the wellbore trajectory . the result is a smaller , more rugged survey instrument that those available in the prior art . attention is directed to fig9 which shows the surface equipment and the downhole instrument probe 20 of the invention . these two basic subsections are connected physically and electronically by means of the wireline cable 114 . the surface equipment will first be discussed . the depth measuring equipment ( dme ) 118 cooperates with a central processing unit ( cpu ) 100 and a recorder 124 . fig9 also shows a surface interface 102 and a surface power supply 104 which provides power to the elements of the surface equipment . a drum 112 stores wireline cable 114 , and deploys and retrieves the cable within the borehole . the cable 114 passes over a measure or sheave well 116 and extends into the wellbore through a set of slips 106 around a pipe 108 . the wellbore is shown cased with casing 110 . the instrument probe 20 , connected to one end of the wireline 114 by means of a cable head 115 , is guided within the casing 110 by a set of centralizing bow springs 130 . the probe 20 encloses an electronic assembly and power supply 132 which powers and controls other elements within the probe . a motor 134 rotates a gyro 136 by means of a shaft 131 . the motor 134 also rotates the accelerometer assembly , shown separately as an x axis component 138 and a y axis component 140 , by means of the shaft 131 . the shaft 131 is terminated at the lower end by a bearing assembly 151 and a lock assembly 153 which fixes the shaft 131 when the drive motor 134 is turned off . probe instrumentation is relatively compact so the length and diameter of the survey probe 20 are relatively small . furthermore , the instrumentation within the probe 20 is relatively simple thereby yielding a very reliable well survey system . other stated objects of the present invention are achieved as discussed in other sections of the above disclosure . attention is directed to fig1 which shows a modified form of instrument . the illustrated portion includes a shaft 231 aligned on the housing centerline and which corresponds to the shaft 131 described with respect to fig9 . the shaft rotates the gyro 236 in the same fashion but the next shaft portion is set at an angle . the angled shaft 239 rotates an accelerometer assembly 238 having the same accelerometers in it as embodiments mentioned earlier . the angle 240 is typically 10 ° to 30 °, the preferred value being about 15 °. the canted angle 240 provides an added data . the unprocessed output of the x and y accelerometers provides two data streams which both can be resolved in two components , one being along the housing or tool axis or centerline 241 ( see fig1 ) and the second resolved component at right angles to the centerline 241 . this angled mounting of the sensors 238 enhances performance by providing more data in vertical well portions . fig1 shows the survey apparatus embodied for measurements - while - drilling . a survey instrument 330 is mounted within a drill collar 324 in the vicinity of a drill bit 320 . the collar 324 and drill bit 320 are suspended from , and conveyed by , a drill string 310 which is made up of sections of drill pipe 308 of length dx which is typically 30 feet . it should be understood , however , that other lengths of drill pipe can be used , and that the lengths of each section of drill pipe does not have to be constant , as long as each section length is known . the drill string 310 is terminated at an upper end by a kelly 300 which is rotated by a rotary table 304 . the rotation of the drill string 310 rotates the drill bit 320 and therefore advances the borehole 322 . since rotary drilling apparatus and methods are well known , other elements of the drilling rigs such as the derrick , mud system and the like required at the surface of the earth 315 are not shown in fig1 . a more detailed view of the mwd survey instrument 330 is shown in fig1 . a motor 334 rotates , by means of a shaft 331 , a gyro package 336 consisting of two orthogonal rate gyros . the motor 334 also rotates , by means of the shaft 331 , an accelerometer assembly 338 which contains an x - axis accelerometer , a y - axis accelerometer , and alternately a z - axis accelerometer as will be discussed in more detail in the following sections . the shaft 331 is terminated at the lower end by a bearing assembly 351 and a lock assembly 353 which fixes the shaft 331 when the drive motor 334 is turned off . power is supplied to all components within the survey instrument by a power supply 332 . as mention previously , considerable vibration is experienced at the drill collar 324 when the drill string is rotating to advance the borehole 322 . referring again to fig1 , a vibration sensor 360 within the survey package 330 monitors the level of vibration at the drill collar . when a low level of vibration is measured , this indicates that the drilling operation has ceased , which typically occurs to add another length of drill pipe 308 of length dx . when this occurs , the vibration sensor activates the motor 334 and initiates the survey measuring sequence . the survey measurement is similar to the wireline surveys discussed previously , but with the major difference being that measurements are made as “ station ” measurements with all instrumentation stationary at a given survey depth within the well borehole . once drilling is again initiated , the vibration sensor 360 measures an increase in vibration and shuts off the motor 334 thereby terminating the measurement cycle . the cycle is repeated as the drill string rotation is again stopped , usually within a depth of dx ( typically equal 30 feet ) to add another length of drill pipe 308 . data from the sensor packages 336 and 338 are transferred to a processor 362 where relative and absolute values of borehole azimuth and inclination are computed for that specific station within the borehole . these results are stored as a function of station depth in a memory 364 for subsequent retrieval when the drill string is removed from the borehole or “ tripped ”. alternately , unprocessed or “ raw ” sensor data can be stored in the memory for subsequent retrieval and processing at the surface 315 . processing of data from the survey instrument in the mwd embodiment is similar to wireline processing previously discussed . since horizontal or near horizontal boreholes are common in mwd measurements , and since the x - axis and y - axis accelerometer outputs are equal and approximately zero in this orientation , an optional z - axis accelerometer is employed to improve the inclination measurement . referring to fig1 , the accelerometer outputs a x , a y , and a z are represented by boxes 408 , 412 and 411 , respectively . the values of the x - axis and y - axis accelerometer responses are compared at step 421 . if the responses are approximately equal and approximately equal to zero , the relatively inclination is set to 90 degrees at step 423 . if either of the x - axis and y - axis accelerometers is non zero , then these measured values are used to form the ratio a x / a y at the step represented by step 422 . the gyro outputs g x and g y , represented by the boxes 400 and 404 , respectively , are combined with this ratio at step 422 to correct the ratio for any non gravity acceleration effects . the computation at step 422 yields the rate of roll over the hsr direction with respect to a reference rate of roll . this quantity is integrated over the depth increment dx , measured from a reference depth x o and combined with g x and g y at step 424 to yield a relative borehole inclination . this relative borehole inclination , when combined with a reference borehole inclination 414 stored in a memory device 420 , yields the desired borehole inclination i c with the system operating in the mwd embodiment . the reference inclination i is determined using techniques used in the previously discussed wireline embodiments . the i c output is represented at 430 . still referring to fig1 , the relative borehole inclination , g x and g y , and a x / a y , are combined and integrated over dx , measured from x o , at step 426 . this yields a relative azimuth value measured at a given station and with respect to a , the reference azimuth 416 stored within the memory 220 . the relative azimuth is combined with the reference azimuth a at step 226 to yield the desired absolute azimuth reading a c , represented at 240 , which in with the azimuth of the borehole computed with the survey system operating at a given borehole station . the reference inclination a is determined using techniques used in the previously discussed wireline embodiments . as discussed previously , i c and a c are combined to yield a map of the borehole in three - dimensional space . in the mwd embodiment , station values of i c and a c are combined with station depths at which they are measured to yield a map of the well borehole . a geometric illustration of such a map is shown in fig1 which uses the same axis convention as used in previously discussed fig2 and 5 . the survey actually begins at a reference depth x o and a reference inclination i and reference azimuth a as illustrated by the point identified as 492 . as each length dx of drill pipe is added , sequential measurements 490 are made and related back to the reference azimuth and inclinations as previously discussed . this process is continued , presumably until the target reservoir is reached as shown in fig1 . it is again emphasized that measurements do not have to be taken at equal depth intervals dx , and the value of dx does not have to be the length of a section of drill pipe . the only requirement is that each sequential depth increment dx is known , and can be algebraically added to the known reference depth x o . stated another way , a c and i c are determined mathematically by summing , over the depth increments dx , measured rates of change of inclination and azimuth with respect to measured , reference azimuth and inclination values . as in the wireline embodiment , this approach greatly simplifies the downhole equipment required to obtain and accurate and precise map of the wellbore trajectory . the result is a smaller , more rugged survey instrument that those available in the prior art . furthermore , each measurement is automatically initiated and terminated by the vibration sensor 360 . while the foregoing is directed to the preferred embodiment , the scope can be determined from the claims which follow .