FR2542455A1 - TRAINING DENSITY LOGGING DURING DRILLING - Google Patents

Abstract

DEVICE AND METHOD FOR DENSITY DIAGRAPHY DURING DRILLING USING A SOURCE OF GAMMA RAYS 22 AND A DETECTOR 24 LOCATED ON A ROTARY ROD TRAIN CONNECTION AND BY CALCULATING THE PRODUCT OF THE ACCOUNTS OBTAINED FROM ' AT LEAST THREE LOCATIONS IN TRAINING SAMPLE 38, THESE LOCATIONS ARE ARRANGED IN A SYMMETRIC AZIMUT PATTERN AROUND THE FITTING, WHICH GIVES THE AVERAGE DENSITY OF THE TRAINING SAMPLE SURROUNDING THE SURVEY. ALTERNATIVELY, THE FITTING INCLUDES THREE PAIRS OF GAMMA RAY SOURCE AND DETECTOR ARRANGED SYMETRICALLY AROUND THE FITTING AXIS AND THE PRODUCT OF THE ACCOUNTS PROVIDED BY THE THREE DETECTORS TO OBTAIN THE AVERAGE DENSITY OF A SAMPLE IS CALCULATED . THE REPORT IS SUITABLE TO MEASURE THE DENSITY OF THE SAMPLE WHATEVER ITS OWN LOCATION IN THE SAMPLING AND THE CHEMICAL COMPOSITION OF THE DISRUPTIVE MATTER INTERPOSED BETWEEN THE TRAINING SAMPLE AND THE DETECTORS.

Description

The present invention relates to logging o-

  carried out on underground formations for the purpose of determining

  density with gamma rays In particular,

  the invention relates to the determination of the density of form

  mation during drilling of a survey crossing the formation

  More specifically, it relates to the determination of

  mination of the formation density operated without placing the logging probe against the borehole wall crossing the

earth formation.

  Density gamma ray probes

  by cable are devices with a radio source

  gamma rays and a gamma ray detector, separated from each other by shielding to avoid counting radiation emitted directly by the source During the operation of

  the probe, gamma rays (or photons) emitted by the sour-

  this penetrate the training to be studied and interact with

  the atomic electrons of the matter of formation by ab-

  photoelectric sorption, by Compton scattering or by pro-

  duction of pairs In photo-absorption phenomena

  electric and pair production, Particu- photons

  links interested in the interaction are eliminated from the bundle

gamma rays.

  In the Compton scattering process, the interested photon loses only part of its energy when changing its initial direction of movement, the loss is a function of the angle of scattering Some of the photons 6 eyes through the source towards inside the sample are consequently scattered towards the detector Many of the scattered rays do not reach the detector, since their direction is changed by a second Compton scattering or they are absorbed due to the photoelectric absorption appearing during the pair production process Scattered photons that reach the detector interact with it are counted by associated electronic equipment

to the detector.

  Among the major difficulties encountered in conventional gamma ray density measurements are the definition of the size of the sample, limitation 2.

  effective depth and sampling, the ef-

  undesirable effects of undesirable disturbing substances

  located between the density probe and the sample and the ne-

  stop placing the probe against the borehole wall The chemical composition of the sample also affects the reading of the results provided by conventional gamma ray density probes These difficulties are even more acute when

  the density measurement tool is incorporated into a ti-

  ge and works during drilling operations There are no

  no known density probes that operate effectively

the measurement during drilling.

  The cable drop density sensor according to the

  Applicant's US Patent 3,202,822 has two detectors

  gamma rays, a source of collimated gamma rays and electronic reporting circuits, and

  is effective as long as the disturbing material located

  the detectors of the probe and the training sample have the same thickness and the same chemical composition along the trajectories of the gamma rays emitted and received O Defects

  of uniformity of the borehole wall impair the proper function-

  of the probe Such inconsistencies can be

  caused by deviated soundings, landslides and va-

  thickness of the mud deposit covering the borehole wall, the prior art also includes US Pat. No. 3,046,631 which describes a density probe lowered by

  cable working regardless of thickness and com-

  chemical position of materials between the denture probe

  sity and the sample, The process consists in making two beams of gamma rays emanating from the sample

  intermittent sources of gamma rays; did-

  re receive the backscattered radiation from each of the two sources by two separate detectors and establish

  product reports of the four separate counting speeds

  so that the numerical result is representative of

the density of the sample.

  The critical dimension of the two-sensor probe

  the spacing between the detectors If the -materials 3.

  are non-uniform over comparative distances

  at the spacing of the two detectors, the density measured

is wrong.

  Neither of the probes lowered by cable

  ble described above is only described as suitable for measurement during drilling and for incorporation in a train of

rotating rods.

  The main object of the present invention is to pro-

  -posing a method and an apparatus for density measurement

  underground formation during the drilling of a borehole

ge crossing training.

  Another object of the invention is to propose an apparatus for logging the density of a formation

  surrounding a survey which crosses it, the apparatus proper to

  be incorporated into a string of rods, comprising: means for the emission of gamma rays towards the interior of the formation; a means of counting emitted-re-scattered gamma rays towards

  the device by a sample located in the formation, the mo-

  counting yen providing an I number of separate accounts,

  each account being determined over a chosen period of time

  sie in advance, the periods being equal and starting at the beginning

  goal of each rotation of 3600 / I, I being an integer equal to at least three; and a means for determining the product of the number I of accounts, this product being representative of the

  average density of the training sample.

  The object of the invention is still achieved thanks to

  a method for determining the average density of a sample

  tillon of a surrounding formation of a borehole which crosses it, comprising the rotation of the device; the emission of gamma rays entering the formation; the determination of a number I of accounts of the gamma rays emitted backscattered by a first training sample to the device,

  each account being determined over a chosen period of time

  sie in advance and starting at the start of each rotation of

  3600 / I, I being an integer equal to at least three, and the determination

  mination of the product from at least three accounts, this product being representative of the average density of the first training sample. 4. Another object of the invention is achieved thanks to

  the apparatus according to the invention which comprises a device for-

  vant to serve in a survey through a training of ter-

  rain, comprising a gamma-ray emission means, this mo-

  yen emitting beams of collimated gamma rays followed

  at least three trajectories, the trajectories are projected-

  both in a symmetrical pattern in azimuth around the axis of the dis-

  positive, intersecting at a first point on the axis of the device

  tif and cutting a first circle located in a sample

  of training subject to the measure, a first means of

  tection of gamma rays oriented so as to receive radiation

  gamma yons emitted, broadcast from at least three places

  elements inside the training sample following a first of at least three trajectories, the trajectories projecting in a symmetrical pattern in azimuth around the axis

  of the device, intersecting at a second point on the axis of the device

  positive and cutting the first circle, and a means to

  finish the product of the counting frequencies of gam-rays

  ma received by the detector means from each of the tra

  at least three jectories as disseminated through

  shooting locations at least three inside the training sample, this product being representative

  the average density of the training sample.

  The object of the invention is still achieved thanks to

  a method for determining the average density of a sample

  field training field surrounding a survey, including

  both the lowering operations of a device in the sound-

  dage to a location near the sample; emis-

  gamma-ray radiation towards the interior of the formation,

  firing of the device, following at least three trajectories projecting in a symmetrical pattern in azimuth around the axis of the

  device, intersecting at a first point on the axis of the device

  positive and intersecting a first circle located in the training sample, the counting of the gamma rays emitted backscattered from the training sample towards the device along a first set of at least three trajectories projecting in a symmetrical pattern in azimuth around the axis of the device, cutting a second point on the axis of the device and cutting the 5. first circle, and determining the product of the measurements

  at least three accounts, this product being

  presentation of the average density of the training sample

  tion. Other features and advantages of the invention

  tion will appear more clearly on reading the description

  detailed non-limiting information, given below of realization

  preferred embodiments of the invention with reference to the accompanying drawings, in which:

  Figure 1 is a cross-sectional representation

  versale of a device according to the present invention, for the density logging of a formation, comprising three sources and three detectors;

  FIG. 2 is a representation in cross section

  side of a device according to the present invention, for logging the densities of a formation crossed by a rotary drill string, in which the device can be

  placed, the device comprising a source and a detector.

  To describe the invention, reference will be made to currently preferred modes of retaliation, but this is for

  in no way limiting, because the invention covers all modifications

  tions and variants falling within its scope as defined by

annulled claims.

  The density detection fitting 10 according to the in-

  vention is shown placed between the upper rod train 12 and the lower rod train 14 which rotates to make

  drill through the drill bit 16 the borehole 18 crossing a form

  field 20 o Connection 10 includes a source of gaïma rays

  52 and a first gamma ray detector 24 o The first

  guard 24 is preferably located on the connector 10 in 1 '-

  azimuth alignment of the source 22 The first detector 24

  is collimated to receive scattered gamma rays from

  firing of the formation following a trajectory 26 which cuts the

  trajectory of gayaa rays emitted from source 22.

  The connector 10 may include a second ganma ray detector 30 This second detector 30 is preferably

  located on fitting 10 in the azimuth alignment of the source

  6. ce 22 and the first detector 24 The second detector 24 receives gamma rays along a trajectory 32 which intersects the trajectory 28 of the gamma rays emitted by the source The trajectory 32 of the rays received by the second detector must be parallel to the trajectory 24 rays received by the

first detector.

  The first and second detectors are shielded from

  so as not to be reached directly by gam- rays

m issued by the source.

  A first circle 34 is drawn by the point of-

  intersection between the trajectory 28 of the rays emitted by the

  source and trajectory 26 of the rays received from the first

  guard, during the rotation of the connector 10 about its axis.

  A second circle 36 is drawn by the point of intersection between the trajectory 2 $ of the rays emitted by the source and the trajectory 32 of the rays received from the second detector, during the rotation of the fitting The first circle is located in a first training sample 33 to be subjected to the density measurement and the second circle 36 is located in the second

  40 so Gis training sample to a measure of establishment

compensated average.

  According to the method of the invention, the rac-

  cord 10 revolves around its are 42 during emission by the source 22 of gamma rays entering the sample The rays of r 7 have ga Ma collimated, emitted define a

  first reg Sion conical in the formation subjected to the ray-

  is lying. In formation @ 20 some of the gamma rays

  emitted 2 L are sent, by diffusion by the sample of form

  mation 3 & location 44, to the first detector 24 and

  counted during a first collection time T 1 Of the

  yons gamma 46 and 43 are sent, by diffusion to the places-

  elements 50 and 52 of the training sample, to the first

  detector 24 and received by it during second and three

  sith collection time, T 2 and T 3 respectively.

  Since there is only one collimated source 22, there is only one region in right cone subjected to radiation

  during rotation of the fitting The collimated detector 24 re-

7.

  receives diffuse gamma rays 26, 46 and 48 from the

  training sample 38 following trajectories which

  have a second cone, turned over from the first cone

  and cutting it along the first circle 34.

  -5 The cone thicknesses are determined by the di-

  source collimator ammeter and first and second detectors The first circle 34 defined by the intersection of the cones at its center on the axis 42 of the connector 10 During a revolution of the drill string, gamma rays emanating from three small sectors 54, 56 and 58 of the training sample 38, each of which is 1200 from the others, are received by the detector 24 and sampled during the

collection T 1, T 2 and T 3.

  Although T 1 = T 2 = T 3 (the collection times are equal), the counting measurements performed during Tl, T 2 and T 3

  may vary in size due to location of connection

  cord and intermediate material.

  Individual accounts from the first and second detectors for a respective collection time can

  vary over time due to the location given to the rac-

  cord in the survey following a rotation of the train

  ges not centered on the survey axis.

  The counting of received gamma rays begins when

  the fitting described an increment of rotation of 3600/1 from

  shot from a starting point and continues to spin for a

  prefixed time period The time measurements performed during

  during the prefixed period of time are all kept se-

  The measurements in number I corresponding to a revolution

  given are multiplied together to give a pro-

  duit, which is representative of the average density of 1

  training sample The time periods are determined

  born, according to the sensitivity of reception and the frequency of-

  emission of the source, so that the number of accounts is representative of the backscattering of rays i from the

training.

  The angle of rotation can be determined by conventional means. In addition, it is possible, without departing from the scope of the invention, to ignore the angle of 8.

  rotation when the angular speed of rotation is relative-

  constant 'In such a case, the counting periods would start at separate time intervals with the

  rotation time divided by the whole number l.

  According to the method of the invention, the instantaneous accounts, at least three, established by the first detector during a revolution are multiplied between them which gives a constant value thus allowing

  to eliminate the variables as a function of the time between a revo-

  lution and subsequent revolutions, such as the thickness of mud that the gamma rays emitted must pass through to be

  received by the detector and that the movement described by the

  cord relative to the borehole wall.

  The fitting, placed in the off center position, will receive

  at the gamma ray detector 24, 26, 46 and 48, different

  rockets from training sample 38, which will have

  crossed different amounts of mud and formation.

  However, the sum of the lengths of path through the mud and the sum of the lengths of path through the formation are constant provided that the diameter of the fitting 10 is substantially identical to the diameter of the borehole 180. As a variant, the fitting 110 has a first source. of gamma rays 122, a second source of gamma rays 124 and a third source of gamma rays 126 Les

  three sources are located around the fitting in a sy-

  metric-in azimuth We can plan more sources as long as they all have a symmetrical arrangement in azimuth around the connector 110 The sources are collimated to define trajectories which are also symmetrical in azimuth The trajectories are oriented so as to pass

  by a first point 128 located on the axis 129 of the connector 110.

  The word trajectory as used here designates not only the actual path traveled by the gamma ray, but also a straight line extending it behind the source as well as beyond the

detector.

  The various sources can be constituted by a single primary source from which the emitted gamma rays are collimated in order to form the beams of 9.

  symmetrical gamma rays at least three in number.

  The connector 110 also comprises a first set of detectors which comprises a first gamma ray detector, a second gamma ray detector 132 and a third gamma ray detector 134 The detectors are located

  turn of fitting 110 in a symmetrical pattern in azimuth and a-

  radial alignment with the first, second and third sources

  these 122, 124 and 126 If the number of sources is greater, we will also use an equivalent number of detectors The detectors are collimated to receive gamma rays scattered from the formation following trajectories which are also symmetrical in azimuth The trajectories are oriented so as to cut the axis 129 of the connector 110 into a

second point 136.

  The trajectories starting from the sources cut the

  first set of detector trajectories on a first circle

  key 138 surrounding the fitting 110 The first circle is located in a plane perpendicular to the axis of the fitting 110, this plane cutting the axis 129 at a third point 139 The second point is at a certain axial distance from the first point and the first and second points are preferably located on the side and

else of the third point 139.

  The connector 110 includes a second set of detectors

  comprising a fourth gamma-ray detector 140, a cin-

  fifth gamma-ray detector 142 and a sixth detector

  of gamma rays 144 This second set of detectors is located

  turn of fitting 110 in a symmetrical pattern in azimuth and also

  in azimuth alignment with the first, second and third

  sources 122, 124 and 126 and the first set of detectors

130, 132 and 134.

  The second set of detectors receives gam-

  ma following a third set of at least three trajectories which are symmetrical in azimuth around the fitting 110 and oriented so as to cut the axis 129 at a fourth point 145 and to cut a second circle 172 Preferably the fourth point 145 and the first point are located on either side of the third point 139 Each trajectory of the third game must be parallel to a corresponding trajectory of the second 10. game.

  The first and second sets of detectors are pro-

  shielded sources so that gamma rays emitted

do not reach them directly.

  The first circle 138 and the second circle 172 define

  finished in training 120 will be the sample centers

  Training time 146 and 147, respectively, to be submitted to

density measurement.

  In the method according to the invention, the connector 10

  rotates around its axis 129 during transmission towards the interior

  laughter of the gamma ray sample 148 by the first source 122, of gamma rays 150 by the second source 124 and

  of gamma rays 152 from the third source 126 The

  these collimated gamma rays emitted define in the form

  a cone-shaped region subject to radiation.

  In formation 120, certain gamma rays 148, and 152 are scattered by sample formation 146 to

  the first set of detectors Gamma rays 154 are diffe-

  rockets at location 156 of training sample 146

  to the first detector 130 and received therefrom

  gamma yons 158 and 160 are scattered at locations 162 and 164 of the training sample 146 to the second and third detectors 132 and 134, respectively, and received by them Since the three collimated sources 122, 124 and 126 are arranged symmetrically, there is only one region in the shape of a right cone subjected to radiation during the rotation of the fitting. The three collimated detectors 130, 132 and 134 receive emitted gamma rays, backscattered from the training sample 146 to the connector 110 following trajectories defining a second cone, returned

compared to the first cone.

  The gamma rays received 154, 158 and 160 act on the first set of detectors 130, 132 and 134 to give

  birth of electrical impulses The amplitudes of

  pulse are proportional to the energy of gamma rays

  If you wish to obtain counting rates represented

  only sensitive to those of the rays which were only diffused once in the sample 134, one can make amplify 11. these impulses by preamplifiers and amplifiers and send them to discriminators not represented, adjusted to let pass only the pulses corresponding to

* energy levels of gamma rays scattered at the site -

  156 towards detector 130, at location 162 towards

  detector 132 and at location 164 towards detector 134.

  Gamma rays having undergone multiple scattering before

  entering the detectors 130, 132 and 134 are found

  thrown by the discriminators The output signals of the

  tectors and, possibly, discriminators reach the doors, not represented, which provide rates of

  individual counting of gamma rays received from the three detectors

130, 132 and 134.

  Individual accounts from the first and second

  cond sets of detectors may vary over time - due to the location given to the fitting in the borehole when the rotation of the drill string is not centered on the borehole axis.

  Depending on the process variant according to the invention

  tion, the three instantaneous accounts emanating from the first set of detectors are multiplied together to obtain a constant value, which eliminates factors varying over time such as the thickness of mud and casing that the

  gamma rays must pass through to be received by detectors

  and that the movement described by the fitting with respect to

the borehole wall.

  A density diagram measured during drilling must be accurate to within ± 0.1 (g / cm 3). Whereas the ground density is typically 2.5, a

  accuracy greater than 4% If the vertical resolution requires

  For the east diagram of 0.15 m, the counting speed required can be estimated as follows:

__ 0.01

N 1 N 2 N 3

I 5

  o is the statistical variation of the product N 1 N 2 N 3, N 1 is the counting speed at the detector 30, 12. N 2 is the counting speed at the detector 32 i and

  N 3 is the counting speed at detector 345.

  If we set N 1 = =, we have: 2 = 3 N 5 and

= = 3 $ 0.01

and by resolution of N

4

N> 3.0 x 10 accounts

  For each density log measurement,

  real detect an average of 30,000 accounts per measurement and it will be necessary to carry out at least one measurement every 15 centimeters A

  a drilling speed of 18 m / h, each measurement will therefore be a-

  chevée in 10 seconds or less, since it must be completed in no more than 1/3 of a revolution Consequently, the first detector, and the second if it is provided for compensation purposes, must have a

  sufficient sensitivity for recording of

  approximately 1,000 per second per detectoro Alternatively, you can

  adjust the source to output at the frequency you want

  read so that detectors receive at the required frequency

  1000 accounts per second per detector.

  Since the fitting is rotating, each measurement counts

  may be less than that required for a static measurement

  tistically precise training density A mode of-

  obtaining precision consists in adding the products

  of each revolution until a number of accounts is obtained

  tes sufficient The summation of products, or product reports in the case of a compensated density, does not give an absolute density but is representative of the density

  taken in correlation with previous measurements.

  To compensate for the average density of the

  training sample 38, the method of the present invention

  tion may include the use of accounts emanating from the

  cond detector 30 during counting periods, on behalf of

  bre of at least three, of a revolution of the drill string On 13. would use the product of these accounts at least three in number to establish a relationship between the product of the first detector and the product of the second detector As a variant, we

  can use the product of the three reports of the first

  second detector to determine the density

average thinking.

  An arrangement a- can be incorporated into the connector 10.

  analog assigned to the second detector 30 for the reception, discrimination, counting, storage and use of

  gamma rays received by the second detector.

  The type of gamma ray source is not 1 '-

  object of the invention, because the preferred type differs from an ap-

  replication to another Sources of the capsule type, containing radioactive isotopes such as cobalt 60 and cesium 137,

  are the most frequently used sources of gamma rays

  in gamma ray density probes.

  The diameters of the borehole and of the fitting 10 must be substantially equivalent. This can be obtained by providing stabilizers outside of the fitting 10.

  this is then taken into account to determine the relative diameter.

  Various other modifications can be made to the structural details and the sequence of calculations

  without departing from the scope of the invention.

14.

Claims (22)

  1 rotary device for use in a borehole passing through a formation in the ground, characterized in that it comprises a means for emitting gamma rays (22), this means emitting a beam of collimated gamma rays following a first trajectory, cutting a first circle (34) located in a sample of the formation to be measured, a first gamma ray detecting means (24) oriented so as to receive emitted gamma rays scattered from places inside said first circle, a counting means said gamma rays received during a number I of prefixed time periods, all the periods being equal and starting at the beginning of each fraction of revolution of 360 / I, I being an integer equal to at least three, and a means of determining   of the product of the I gamma ray counting frequencies   gus by the first detector means, this first product being   representative of the average density of said sample of form mation (38).   2 Device according to claim 1, character-   in that said device works regardless of its location in the borehole (18)   3 o Device according to claim 1, character-   in that the diameter of said device is substantially equivalent to, but less than, the diameter of the borehole (18),   4 Device according to claim 1, character-   rised in that it further comprises a second detector means (30) oriented so as to receive gamma rays scattered from places inside a second circle (36) which is cut by the path of the gamma rays emitted and a way   for determining a second product of the comp frequencies   tage of gamma rays received by said second detector means   during said prefixed time periods, said first   mier product being divided by said second product for the   determination of a ratio which is representative of a density   average composite tee of said training sample (38).   Device according to claim 4, characteristic   se in that the trajectory of the gamma rays received by the se-   cond medium detector (30) is parallel to that of the rays 15.   gamma received by the first detector means (24).   6 Method for determining the average density of a training sample (38) surrounding a survey (18)   which crosses said formation, characterized in that it comprises   carries the rotation of said device; the emission of gamma rays towards the interior of said formation, the determination   of a number I of accounts of said gamma rays emitted retrodif-   rockets from a first training sample to   said device, each account being determined on a   time period prefixed and starting at the beginning of each fraction of revolution of 3600/1, I being an integer equal to at least three, and the determination of a first product of said accounts at least three in number, said first   product being representative of the average density of said é- training sample.   7 Method according to claim 6, characterized   in that at least three separate accounts are determined for-   said gamma rays emitted backscattered from a second training sample to the device, each of said   accounts being determined during said time period cor-   responding to said accounts at least three of the first   first training sample, a second product of accounts at least three is determined and the ratio between said first and second products is determined, this ratio   being representative of an average compensated density of the training sample.   8 Method according to claim 6, characterized in that it is implemented without placing said device in the poll.   9 Method according to claim 7, characterized in that the trajectories of backscattered rays from   of the first training sample are parallel to the tra-   jectories of rays backscattered from the second sample training tillon.   10 Method according to claim 6, characterized in that further collimates the rays emitted from the device for obtaining a beam whose trajectory   cuts said first and second training samples.   16. 11 Method according to claim 6, characterized   in that the device is further collimated so that it   only gauma rays diffused 6 from the first sample. 12 Method according to claim 7, characterized   in that the device is further collimated so that it   only gamma rays scattered from the second sample. 13 Apparatus for the density logging of a formation surrounding a borehole which crosses the formation, this apparatus being suitable for use in a drill string and characterized in that it comprises: a means for counting emitted gamma rays backscattered from of a sample of   training towards the device, this counting means provided   sant a number I-of separate accounts, each account being determined during a prefixed period of time, all the periods being equal and starting at the beginning of each fraction of revolution of 360 / I, I being an integer equal to   at least three; and a means for determining a pre-   mier product of said number I of accounts, this first product being representative of the average density of the sample training.   14 Apparatus according to claim 13, character-   se in that said counting means is oriented so as to detect gamma rays scattered along a path which   cuts said training sample.   Apparatus according to claim 13, character se in that I is equal to 3.   16 Device for use in a borehole passing through a formation in the ground, characterized in that it comprises a means for emitting gamma rays (122, 124, 126), this means emitting beams of gamma rays collimated according to a first set of '' at least three trajectories, the trajectories projecting in a symmetrical pattern in azimuth around the axis of the device, intersecting at a first point (128) of the axis of the device and intersecting a first circle (138) located in a sample of the formation subjected to the measurement, a first means of detecting gamma rays (130, 17.  132 "134) oriented so as to receive emitted gamma rays scattered from at least three locations inside the training sample in a second set of at least three paths, the paths projecting in a symmetrical pattern in azimuth around the axis of the device, intersecting at a second point (136) of the axis of the device and intersecting the first circle, and means for determining the product of the counting frequencies of gamma rays received by   the first detector means from each of the trajectories   at least three in number, as broadcast from   locations with at least three interiors within 1 '-   training sample, this product being representative of   the average density of the training sample.   17 Device according to claim 16, character-   laughed in that said first circle (138) extends in a plane perpendicular to the axis of the device and intersecting this axis in a third point (139).   18 Device according to claim 16, charac-   in that the device is adaptable for setting work in a drill string.   Device according to claim 16, character & -   laughed at in that said gamma ray emission means   takes sources of gamma rays (122, 124, 126), each of which   none is collimated to emit gamma rays along one of said trajectories at least three in number first game.   Device according to claim 19, charac-   terized in that said sources are distributed in a symmetrical pattern in azimuth around the device and located in a   second plane which is perpendicular to the axis of the device.   21 Device according to claim 16, charac-   terized in that said first detection means comprises at least three detectors (130, 132, 134), each of which is   collimated to receive gamma rays according to one of the   say at least three trajectories from the second game.   22 Device according to claim 16, character-   laughed at in that it works regardless of its location in the poll. 18.   23 Device according to claim 16, character-   laughed at in that its diameter is substantially equivalent but   slightly smaller than the borehole diameter (118).   24 Device according to claim 16, charac-   terized in that it further comprises a second means of   tection (140, 142, 144) oriented to receive gamma rays   ma emitted broadcast from said locations at least three inside the training sample (138) following a third set of Vau minus three trajectories, these trajectories intersecting at a fourth point (145) of the axis of the device and intersecting a second circle (172) surrounding this axis, this circle 6 being cut by the first set of trajectories at least three in number, and a means for determining a   second produces frequencies of gamma ray coiptage re-   received by the second detection means from each of the   say trajectories numbering at least three, such as diff-   rockets from each of said locations at least three in number, inside the training sample (138), said first product being divided by said second product   for determining a ratio that is representative of-   an average compensated density of the training sample.   Device according to claim 16, character-   risâ in that l said first point (128) is' spaced from said se- cond point (13 e).   26 o Device according to aeevndication 24, charac-   terized in that the first point (12 e) and the second point (136) are spaced from the fourth point (145) o   28 Device according to claim 26, character-   laugh that the first E point (128) is located on one side of the SO 30 third point (139) and the second point (136) is located   opposite side of said third pointo.   29 Method for determining the average density   ne of a foription sample eztourant a survey, ca-   characterized in that it includes the operations of lowering   a device in E probing to a location close to the sample; emission of gamma rays towards the interior of the formation, from the device, following at least three trajectories projecting in a symmetrical pattern in 19.   azimuth around the axis of the device, intersecting in a pre-   point of the axis of the device and cutting a first circle   key located in the training sample, counting the gamma rays emitted backscattered from the training sample to the device according to a first set of at least three trajectories projecting in a symmetrical pattern in azimuth   around the axis of the device, cutting a second point by 1 '-   axis of the device and bending the first csrcle, and determining it   mination of the product of said accounts at least three in number, this product being representative of the average density of the training sample.   Method according to claim 29, characteristic   se in that it is implemented without placing said device in the poll.   31 Method according to claim 29, character-   se in that it still includes the counting of said rays   gamma emitted backscattered from the training sample   tion to the device according to a third set of at least   three trajectories projecting into a symmetrical pattern in a-   zimut around the axis of the device, intersecting in three   sixth point spaced from said second point located on the axis of the dis-   positive and intersecting said second circle around said axis, the-   said circle being cut by said first set of trajectories at least three in number; the determination of said second   generates at least three accounts, and deter-   reduction in the relationship between said first and second products,   said ratio being representative of an average density   thought of the training sample.