CA2154378C

CA2154378C - Method and apparatus for interrogating a borehole

Info

Publication number: CA2154378C
Application number: CA002154378A
Authority: CA
Inventors: Larry W. Thompson; Macmillan M. Wisler
Original assignee: Baker Hughes Inc
Current assignee: Baker Hughes Holdings LLC
Priority date: 1994-08-01
Filing date: 1995-07-21
Publication date: 2006-03-21
Anticipated expiration: 2015-07-21
Also published as: EP0704717A2; EP0704717A3; NO952984D0; NO315955B1; CA2154378A1; EP0704717B1; NO952984L; US6288548B1

Abstract

A method and apparatus is disclosed for performing measurement-while-drilling operations in a borehole to interrogate the borehole and the surrounding formation utilizing electromagnetic fields. A
measurement sonde is provided which includes at least one of (a) a transmitting member for generating an interrogating electromagnetic field for passage through the borehole and surrounding formation, and (b) a receiving member for receiving an interrogating electromagnetic field after passage through said borehole and surrounding formation. A measurement tubular is provided, which includes a central bore which communicates with the central bore of the drillstring. The measurement tubular couples in the drillstring in a selected location, and permits interrogation of the borehole and surrounding formation with the interrogating electromagnetic fields generated by the measurement sonde. Preferably, the measurement tubular is formed at least partially of a non-conducting or poorly-conducting material which will allow the inward and outward passage of electromagnetic fields.
The measurement sonde is secured within the central bore of the measurement tubular. Preferably, it is releasably secured therein, and can be run in and out of the wellbore during drilling operations without substantial interference with drilling operations.

Description

2 1. Field of the Invention:

3 The present invention relates in general to measurement-while-4 drilling systems, and in particular relates to measurement-while-drilling systems which transmit or receive electromagnetic fields.
6 2. Description of the Prior Art:
7 Measurement-while-drilling systems are now essential 8 components of sophisticated exploration operations, and are utilized to 9 provide real-time data pertaining to drilling conditions as well as the wellbore and surrounding formations. In particular, resistivity logs can be developed 11 during drilling operations which, in some cases, provide a full substitute for 12 the more conventional wireline electric logs. Other types of logging 13 operations are also conducted in measurement-while-drilling systems, 14 including neutron porosity measurements which provide a measure of the wellbore and formation porosity. The resistivity measurements can be 16 utilized to provide a measure of the formation resistivity, the borehole 17 diameter, the diameter of invasion of drilling mud into the formation, and to 18 detect formation boundaries and formation changes.
19 In prior art systems, it is conventional to place the transmitting and receiving antennas of the measurement-while-drilling system within the 21 tubular body of a drill collar or drill pipe member. Most commonly, a portion 22 of the drill collar or drill pipe is "necked-down" (that is, milled or cut to a 23 reduced radial dimension), to provide a relatively safe location for placement 24 of the transmitting and receiving antennas. This, of course, structurally weakens the drill pipe or drill collar member, and renders it more susceptible DOCKET NO. 414-4780-US

-3- _ 21~43'~3 1 to mechanical failure during drilling operations. In the prior art devices, the 2 antennas are placed in or about the exterior surface of the drill pipe or drill 3 collar member, since the drill pipe or drill collar member is typically formed 4 from steel. Electromagnetic radiation cannot effectively be transmitted through steel tubulars, since the steel is a highly conductive material, and 6 since the electromagnetic fields generate eddy currents in the conductive 7 material- which dissipate the field. The required exterior placement of the 8 transmitting and receiving antennas exposes the antennas to the not-9 insubstantial forces which arise as the drill pipe or drill collar drags or otherwise engages the wellbore wall. Additionally, cuttings from the 11 formation are circulated upward through the annulus between the drillstring 12 and the wellbore. Cuttings which are propelled at high velocities can 13 become lodged in the antenna assemblies and/or otherwise damage the 14 antenna assemblies through abrasion. Additionally, the construction costs of a tool which has antennas which are part of the collar are high, since an 16 expensive steel drill collar is machined with features which provide for the 17 placement of antenna parts and for housing of the electronics which 18 communicate with the antennas. Maintenance costs are also high for the 19 prior art devices, especially since the tools include a heavy collar which can not be transported easily and since the tool can not be separated easily for 21 replacement or servicing.

DOCKET NO. 414-4780-US

2 It is one objective of an aspect of the present invention to 3 provide an improved measurement-while-drilling system which utilizes a 4 tubular which is at least partially formed of a material which is non-conducting or poorly-conducting, and thus which allows the passage of 6 electromagnetic fields both radially inward and radially outward relative to 7 the measurement tubular.
g It is another objective of an aspect of the present invention to g provide a measurement-while-drilling system which utilizes a transmissive measurement tubular which allows for the passage of electromagnetic fields 11 through the tubular body, in combination with a measurement sonde which 12 is disposed within the central bore of the measurement tubular, and which 13 can be utilized to transmit interrogating electromagnetic signals into the 14 formation and receive electromagnetic radiation from the borehole and surrounding formation to measure one or more borehole or formation 16 characteristics.
17 These and other objectives are achieved as is now described 18 in' the:: conteact of formation resistivity measurement operations. When 19 characterized as an apparatus, the present invention is directed to a measurement-while-drilling apparatus for use in a drillstring during drilling 21 operations to interrogate a borehole and surrounding formation: The 22 measurement-white-drilling apparatus includes a number of components 23 which cooperate. A measurement sonde is provided and includes a 24 transmitting member for generating an interrogating electromagnetic field for passage through the borehole and surrounding formation, and a receiving 26 member for receiving an interrogating electromagnetic field after passage -5- ' 1 through the borehole and surrounding formation. A measurement tubular 2 is also provided which includes a central bore which communicates with a 3 central bore of the drillstring. A measurement tubular couples in the 4 drillstring to locate the measurement sonde in a particular position, and to permit interrogation of the borehole and surrounding formation with the 6 interrogating electromagnetic field. A means is provided for securing the 7 measurement sonde in a particular location within the central bore of the 8 measurement tubular. The measurement-while-drilling apparatus is operable 9 in at least a transmission mode of operation and a reception mode of operation, which preferably occur simultaneously. During transmission 11 operations, the interrogating electromagnetic field is generated by the 12 measurement sonde and radiated outward from the measurement sonde 13 and through the measurement tubular into the borehole and surrounding 14 formation. During reception operations, the interrogating electromagnetic field passes from the borehole and surrounding formation through the 16 measurement tubular for detection by the receiving member. In the 17 preferred embodiment transmission and reception operations occur 18 simultaneously.
19 In one particular embodiment, the measurement tubular comprises a solid cylindrical tubular formed of either non-conducting or 21 poorly-conducting material which allows for the inward and outward passage 22 of electromagnetic fields. In another embodiment, the measurement tubular 23 includes a plurality of axial slots in a steel skeletal structure which are filled 24 with non-conducting or poorly-conducting material, which allow for the passage of the interrogating electromagnetic field from the central bore of 26 the measurement tubular to the borehole and surrounding formation. The 27 most typical application of the present invention requires that the DOCKET NO. 414-4780-US

2.~~437g -s=
1 measurement tubular be a drill collar member for coupling at a lowermost 2 portion of a drillstring.
3 In the preferred embodiment of the present invention, the 4 measurement sonde includes a retrieval member which allows for (1) retrieval of the measurement sonde from the measurement tubular drilling 6 operations without substantial interference with the drilling operations, and 7 (2) placement of the measurement sonde in the measurement tubular during 8 drilling operations without substantial interterence with drilling operations.
9 When characterized as a method, the present invention is directed to a method of interrogating a borehole and surrounding formation 11 during drilling operations, and includes a number of method steps. A
12 measurement tubular is provided which is formed at least partially of a 13 material which allows substantially unimpeded passage of electromagnetic 14 fields, and which includes a central bore. A measurement sonde is provided. The measurement sonde includes a transmitting member for 16 generating an interrogating electromagnetic field for passage through the 17 borehole and surrounding formation, and a receiving member for receiving 18 an interrogating an electromagnetic field after passage through the borehole 19 and surrounding formation. The measurement sonde is secured within the central bore of the measurement tubular. The measurement tubular is 21 coupled in a selected location within a drillstring. The measurement sonde 22 is utilized during drilling operations to interrogate the borehole and 23 surrounding formation, by operating in a transmission mode of operation 24 and a reception mode of operation. During a transmission mode of operation, the interrogating electromagnetic field is generated by the 26 measurement sonde and radiated outward from the measurement sonde DOCKET NO. 414-4780-US

1 and through the measurement tubular into the borehole and surrounding 2 formation. During a reception mode of operation, the interrogating 3 electromagnetic field passes from the borehole and surrounding formation and 4 through the measurement tubular for detection by the receiving member.
6 Accordingly, in one aspect of the present invention there is provided 7 a measurement-while-drilling (MWD) apparatus for use with a drillstring during 8 drilling operations for obtaining information about a parameter of interest, 9 comprising:
a measurement tubular that includes a central bore that 11 communicates with a central bore of said drillstring;
12 a measurement sonde located within the measurement tubular for 13 obtaining measurements on the parameter of interest, said measurement sonde 14 including:
at least one transmitter for inducing an electromagnetic signal 16 in the formation through the measurement tubular; and 17 at least one receiver for receiving the induced 18 electromagnetic signal in the formation through the measurement tubular;
and 19 a retrieval member on the measurement sonde adapted to facilitate an operation selected from (i) retrieval of the measurement sonde from the 21 measurement tubular during drilling operations, and, (ii) placement of the 22 measurement sonde within the measurement tubular during drilling operations.

-7a-1 According to another aspect of the present invention there is 2 provided a method of obtaining measurements relating to a parameter of interest 3 of a formation surrounding a borehole drilled by a drillbit conveyed on a drilling 4 tubular, the method comprising:
detachably coupling a measurement sonde within a measurement 6 tubular having a central bore in communication with a central bore of said drilling 7 tubular;
8 activating a transmitter on the measurement sonde thereby inducing 9 an electromagnetic signal in the formation through the measurement tubular;
and detecting with a receiver on the measurement sonde a detected 11 signal indicative of the induced electromagnetic signal and the parameter of 12 interest.

14 Additional objectives, features and advantages will be apparent in the written description which follows, but one principle advantage includes the 16 ease of separation of the drill collar part of the logging tool from the sonde part of 17 the logging tool. Thus the sonde part can be manufactured, tested, transported, 18 and repaired independently of the collar in which it is housed during 19 measurement-while-drilling operations.

_215437 2 The novel features believed characteristic of the invention are 3 set forth in the appended claims. The invention itself, however, as well as 4 a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an 6 illustrative embodiment when read in conjunction with the accompanying 7 drawings, wherein:
8 Figure 1A is a simplified depiction of a drilling rig, a drillstring 9 and a wellbore equipped with an apparatus for interrogating the borehole in accordance with the present invention;
11 Figure 1 B is a partial longitudinal section view of a 12 measurement tubular and measurement sonde in accordance with the 13 present invention;
14 Figure 1C is a simplified schematic view of the antenna arrangement of the measurement sonde of Figure 1 B;
16 Figure 1 D is a schematic depiction of the operation of the 17 transmitting and receiving antennas;
18 Figure 1 E depicts the phase shift which is detected by the 19 receiving antennas;
Figure 1 F is a graph which depicts the relationship between 21 resistivity and the detected phase shift;
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_g_ _ 21543'8 1 Figure 1 G graphically depicts the amplitude attenuation of the 2 interrogating signal;
3 Figure 1 H depicts the graph of the relationship between 4 resistivity and the amplitude ratio;
Figures 2A through 2E depict the electronic components of 6 the logging apparatus in accordance with the present invention;
7 Figures 3A and 3B depict one particular embodiment of the 8 present invention;
9 Figures 4A, 4B, 4C, and 4D depict another embodiment of the present invention.
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-10- _ 2.~543~$

2 With reference to Figure 1A, there will now be described an 3 overall simultaneous drilling and logging system in accordance with one 4 preferred embodiment of the present invention that incorporates an electromagnetic wave propagation (EWP) resistiviiy measurement system 6 according to this invention.
7 A well 1 is being drilled into the earth under control of surface 8 equipment including a rotary drilling rig 3. In accord with a conventional 9 arrangement, rig 3 includes a derrick 5, derrick floor 7, draw works 9, hook 11, swivel 13, kelly joint 15, rotary table 17, and drill string 19 that includes 11 drill pipe 21 secured to the lower end of kelly joint 15 and to the upper end 12 of a section of drill collars including an upper drill collar 23, an intermediate 13 drill collar or sub (not separately shown), and a lower drill collar 14 measurement tubular 25 immediately below the intermediate sub. A drill bit 26 is carried by the lower end of measurement tubular 25.
16 Drilling fluid (or "mud", as it is commonly called) is circulated 17 from a mud pit 27 through a mud pump 29, past a desurger 31, through a 18 mud supply line 33, and into swivel 13. The drilling mud flows down 19 through the kelly joint and an axial tubular conduit in the drill string, and through jets (not shown) in the lower face of the drill bit. The drilling mud 21 flows back up through the annular space between the outer surface of the 22 drill string and the inner surface of the borehole to be circulated to the 23 surface where it is returned to the mud pit through a mud return line 35. A
24 shaker screen (not shown) separates formation cuttings from the drilling mud before it returns to the mud pit.
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1 The overall system of Figure 1 uses mud pulse telemetry 2 techniques to communicate data from downhole to the surtace while drilling 3 operation takes place. To receive data at the surface, there is a transducer 4 37 in mud supply line 33: This transducer generates electrical signals in response to drilling mud pressure variations, and these electrical signals are 6 ~ transmitted by a surface conductor 39 to a surface electronic processing 7 (SEP) system 41.
8 As explained in U.S. Patent No. 4,216,536 to More (More 9 '536 patent), mud pulse telemetry techniques provide for communicating data to the surface about numerous downhole conditions sensed by well 11 logging transducers or measurement systems that ordinarily are located 12 on and within the drill collar nearest the drill bit. Measurement tubular 13 is preferably nearest the drill bit, as shown in Figure 1. The mud pulses 14 that define the data propagated to the surface are produced by equipment ~5 within the intermediate sub. Such equipment suitably comprises a 16 pressure pulse generator operating under control of electronics contained 17 within an instrument housing to allow drilling mud to vent through an 1g orifice extending through the logging collar wall. Each time the pressure 19 pulse generator causes such venting, a negative pressure pulse is transmitted to be received by surface transducer 37. An alternative 21 conventional arrangement generates and transmits positive pressure 22 pulses.
23 The circulating drilling mud provides a source of energy for a 24 turbine-driven generator sub-assembly located in the intermediate sub; and, 25 the turbine-driven generator sub-assembly generates electrical power for the 26 pressure pulse generator and for various circuits including circuits forming _214378 1 part of the preferred embodiment of this invention. As an alternative or 2 supplemental source of electrical power, batteries may be provided, 3 particularly as a back-up for the turbine-driven generator.
4 A measurement system embodying the present invention includes electronics contained in electronics housings contained within 6 measurement sonde 27, and contains elements arranged in recesses or 7 necked-down portions of the tubular steel housing of measurement sonde 8 27. Some of these elements of measurement sonde 27 are indicated 9 schematically in Figure 1 C and include upper transmitting antenna 29, lower transmitting antenna 31, and intermediate receiving antennas 33, 35 which 11 are carried about an exterior surface of measurement sonde 27, and which 12 are utilized to interrogate the borehole and surrounding formation, as will be 13 discussed in greater detail herebelow. In alternative embodiments, a greater 14 or lesser number of transmitting or receiving antennas may be utilized.
Figure 1 B depicts one embodiment of measurement tubular 16 25, which includes upper, internally threaded tool joint 37 and lower, 17 internally threaded tool joint 39, which are adapted to couple within a 18 drillstring, with a central section 41 disposed therebetween which is formed 19 of a material which allows the inward and outward propagation of electromagnetic fields, to allow the transmitting antennas 29, 31 and 21 receiving antennas 33, 35 of measurement sonde 27 to communicate with 22 the surrounding borehole and formation. In accordance with the preferred 23 embodiment of the present invention, central section 41 is formed of a 24 material which is either "poorly-conducting" or "non-conducting". For purposes of this disclosure, semi-conductors are defined as materials which 26 have a bulk resistivity value of greater than 0.001 Ohm-meters and less than DOCKET NO. 414-4780-US

-13- _ 21~43'~8 1 100 Ohm-meters. For purposes of this disclosure, non-conducting materials 2 are defined as those materials which have bulk resistivity values which are 3 greater than 100 Ohm-meters. Also, for purposes of this disclosure, "good"

4 conducting materials are defined as having a resistivity of less than 0.001 ohm-meters. Central section 41 need merely be sufficiently strong to 6 provide mechanical strength and convey wellbore fluids, but while also 7 allowing electrical sensors located within the interior of measurement tubular 8 25 to transmit and receive oscillating electric and/or magnetic fields which 9 are too high in frequency to penetrate the conventional prior art steel drill collars. As stated above, the prior art steel collars responded to high 11 frequency electric and/or magnetic oscillating fields by the generation of 12 eddy currents which dissipated the field and prevented the communication 13 inward or outward of electric and/or magnetic oscillating fields. Preferably 14 central section 41 may be composed of KEVLAR-based composite materials. An example of one type of composite tubulars which are 16 currently being utilized in the oil and gas industry are the composite drill 17 pipe; casing pipe, and tubing pipe manufactured by Brunswick Composites, 18 a unit of the Brunswick Technical Group, having a business and 19 correspondence address in Lincoln, Nebraska, which offers for sale composite tubulars which have a strength many times greater than that 21 found in steel tubulars, with much less weight, and virtual immunity to 22 corrosion. An article entitled "Developments in Composite Structures for the 23 Offshore Oil Industry" by J. G. Williams of Conoco, Inc., published in May 24 of 1991 at the Offshore Technology Conference, and identified by OTC No.

6579, provides a detailed statement of the current utilization of composite 26 materials in offshore oil and gas activities. Among the numerous uses of 27 composite materials identified in this article is the use of composite drill pipe 28 which has demonstrated its ability to withstand the forces encountered DOCKET NO. 414-4780-US

_14_ _215~3'~
1 during drilling operations. Numerous composite materials are identified in 2 this article including composites based upon graphite, KEVLAR 29, and 3 KEVLAR 49.
4 In the present invention, since only azimuthal transmission of the electromagnetic wave is desired, the measurement tubular need only be 6 transmissive of electromagnetic waves which are traveling inward relative to 7 the measurement sonde or outward relative to the measurement sonde.
8 The measurement tubular need not be transmissive of electromagnetic fields 9 along its central longitudinal axis. In the embodiment of Figure 1 B, the entire measurement tubular is constructed of a material which is 11 transmissive of electromagnetic fields; however, in alternative embodiments, 12 such as those depicted in Figures 3A, 3B, 4A, 4B, 4C, and 4D, only 13 portions of the measurement tubular need be formed of a material which 14 allows the inward and outward passage of electromagnetic fields. In the particular embodiments depicted and described herein, a plurality of axial 16 slots may be formed in a conventional steel tubular. The axial slots are filled 17 with composite material which is transmissive of electromagnetic fields. Of 18 course, the measurement sonde and the measurement tubular should be 19 aligned so that the antenna is placed proximate the axial slots, and allowed to either transmit electromagnetic fields outward through the axial slots, or 21 to receive magnetic fields which are traveling radially inward through the 22 axial slots.
23 The operation of measurement sonde 27 is depicted in 24 schematic and graphical form in Figures 1 D, 1 E, 1 F, 1 G, and 1 H. As is depicted in schematic form in Figure 1 D, transmitting antennas T~, T2 are 26 spaced on both sides of a pair of receiving antennas R~, R2, allowing the DOCKET NO. 414-4780-US

1 measurement of both phase and amplitude. The transmitters and receivers 2 are simple antennas consisting of a loop of wire imbedded in an insulating 3 material, with tuning capacitors to trim the antenna response. The tool can 4 be thought of as a pair of sensors, whose output is the average of two readings, reducing tool error from the temperature and pressure affects, tool 6 misalignment, borehole washout and bed shoulder effects. In a vacuum, 7 the finite velocity of electromagnetic radiation leads to a small phase shift 8 between the two receivers R,, R2, while the increased distance to the far 9 receiver causes the signal strength to be weaker at that point. In a more conductive formation, the radiation moves more slowly and attenuates more 11 rapidly. As a result, either the difference in phase between the two 12 receivers, or the ration of amplitudes may be used to measure formation 13 resistivity. As is shown in Figure 1 D, two amplitude and phase readings are 14 made, and then averaged. An upper transmitter reading X2 is made utilizing upper transmitting antenna T2 to propagate an electromagnetic wave 16 outward through measurement tubular 25 (which is not depicted) and into 17 the formation, and then back through measurement tubular 25 to be 18 measured by the intermediate receiving antennas R,, R2. Then, the lower 19 transmitter reading X, is made utilizing lower transmitting antenna T, to send an electromagnetic wave outward through measurement tubular 25 (not 21 depicted) into the formation, and then back through measurement tubular 22 25 to be measured by intermediate receiving antennas R~, R2. The 23 compensated reading is the arithmetic average of lower transmitter reading 24 X~ and upper transmitter reading X2.
In the preferred embodiment, the primary measurement is 26 phase resistivity which can be described with reference to Figure 1 E. It is 27 easy to see from this illustration that the greater the spacing between the DOCKET NO. 414-4780-US

21543'~~

1 two receiving antennas R~, R2, the larger the observed phase difference 2 between the receiving antennas. The phase measurement is converted to 3 resistivity, with only small effects from the dielectric constant, after averaging 4 the values from both transmitters. Figure 1 F is a graph which plots resistivity in Ohms-meter to phase difference in units of degrees. A family 6 of curves is shown for different relative dielectric constants (for the specific 7 dielectric constant values of 1, 10, and 100).
8 Figure 1 G depicts the amplitude attenuation which occurs as 9 the electromagnetic wave propagates through the formation. A two megahertz wave, under downhole conditions, propagates only a few feet 11 before the signal strength fades away, so the rate of attenuation or 12 amplitude ratio of the signal is also measured. Figure 1 G graphically 13 depicts the attenuation of the signal from its original signal strength as it 14 passes through measurement tubular 25 and the surrounding formation.
Measurements made at receiving antennas R,, R2 can be used to develop 16 an amplitude ratio which is representative of the amount of attenuation that 17 the interrogating signal experiences as it travels between receiving antenna 18 R~ and receiving antenna R2. In the preferred embodiment of the present 19 invention, the tool utilizes a microprocessor with memory to store values of the amplitude for each transmitter into memory, and then computes the ratio 21 of amplitudes for each transmitter, averaging the values for each transmitter 22 to produce a compensated ratio. The amplitude ratio is then transformed 23 to resistivity. Figure 1 H is a graph which plots the relationship of resistivity 24 in units of Ohms-meter to the amplitude ratio, for a plurality of differing relative dielectric constants (and specifically for the relative dielectric 26 constant values of 1, 10, and 100).
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_21437$

1 The following discussion illustrates how the dual transmitter, 2 dual receiver measurement-while-drilling apparatus of the present invention 3 is utilized to derive an accurate measure of the amplitude attenuation and 4 phase shift of the interrogating electromagnetic signal which travels through the borehole and surrounding formation.
6 First, consider four transmitter-to-receiver signals:
7 (Transmitter 1 [X1] to Receiver 1 [R1]): A,~ e~"
8 (Transmitter 1 [X1 ] to Receiver 2 [R2]): A~2 e~'2 9 (Transmitter 2 [X2] to Receiver 1 [R1]): A2~ e~2' (Transmitter 2 [X2] to Receiver 2 [R2]): A~ e~~
11 The measured amplitudes are made up of:
12 Amn = Xm Rn a~n (Eq. 1.1 ) 13 where Xm = transmitter output variation 14 Rn = receiver sensitivity variation au"n = true amplitude (transmitter M to receiver N);
16 and the measured phases are made up of:
17 ~ mn = ~ xm + ~ Rn ~' ~ tmn (Eq. 1.2) 18 where ~ xt, = transmitter phase (output) variation 1 9 ~ ~, = receiver phase variation ~ tmn = true phase (transmitter M to receiver N) DOCKET NO. 414-4780-US

_ 18 __ 21543'~~
1 The foregoing general equations correspond to the following 2 more specific equations:
3 A,~ = X~ R~ a~"

4 A,2 = X, R2 aC,2 A2~ = X2 R~ ate, 6 A~ = X2 R2 a~

7 ~ =~x~ +~R~ +~tm ~ ~2 = ~x~ + ~ R2 + ~c~2 'r21 -~X2 +~R1 +~'t21 ~~ _~~+~~+~~

11 Taking rations of the various transmitter-to-receiver signals 12 produces the following:
13 For Transmitter 1:
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_ 19 _ _ 214378 _________ _ ____ a ~~,2 -~ ") and for Transmitter 2:
6 ___'_____ _ ____* a ~E 2, -~ ~) Multiplying these and taking the square root gives:
At2 * e1 ~1t2 - ~tt~ * ~ * e~ib2t - ~
~11 ~12 * A21 e! Y~(~tt ~ ~2t - ~tt - ~
"t 1 * ~2 Straightforward algebraic manipulation of Eqs. 1.1 through 1.3 yields:
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_ 2154~'~$

$hp * 8~' 1 * efYs(~t12 i;t21 -;t11 - ~t2~
8t11 * 8t22 1 because all the system variables drop out of the measurement.
2 Therefore, by using two transmitters and two receivers, 3 systematic variables can be removed from both the attenuation (amplitude) 4 and from the phase velocity (phase difference) terms.
Within the context of the preferred embodiment of this 6 invention, in which a sampled-data processing means produces a signal as 7 a function of formation resistivity based on phase-representing signals, the 8 following analysis demonstrates certain matter relevant to the stability 9 feature.
Consider two consecutive samples: Sample A and Sample B.
11 During Sample A, a first transmitting coil is energized to cause 12 a wave to propagate through the formation in a direction such that the wave 13 passes a first receiving coil (R1), and later passes a second receiving coil 14 (R2), and induces each receiver coil to produce a signal.
During Sample B, a second transmitting coil is energized to 16 cause a wave to propagate through the formation in a direction such that 17 the wave passes a second receiving coil (R2), and later passes the first 18 receiving coil (R1), and induces each receiver coil to produce a signal.
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215437$
_21 _ _ 1 Let ~ MR2A represent the measured phase of the signal 2 produced by receiver coil R2 during Sample A; let ~ MR1A represent the 3 measured phase of the signal produced by receiver coil R1 during Sample 4 A; let ~ MR1 B represent the measured phase of the signal produced by receiver coil R1 during Sample B; and let ~ MR2B represent the measured 6 phase of the signal produced by receiver coil R2 during Sample B.
7 The ~ MR2A signal depends on the phase of the wave at the 8 location of R2, and in general, has an error component attributable to 9 various phase shifts including those introduced by the tuned receiver coil, cabling from the receiver coil to the receiver, and the receiver itself. Let 11 ~ TR2A represent the true phase of the wave at the location or R2 during 12 Sample A, and let c~ R2E represent the error component so introduced.
13 Eq. 2.1: ~ M R2A = ~ TR2A + ~ R2E
14 Similarly, they MR1A signal depends on the phase of the wave at the location or R1, and in general, has its own error component. Let 16 ~TR1A represent the true phase of the wave at the location of R1 during 17 Sample A, and let ~ R1 E represent the error component so introduced.
18 Eq. 2.2: ~MR1A =~TR1A +~R1E
19 During Sample A, the ~ MR1A signal and the ~ MR2A are simultaneously processed to produce a DeItaA signal that represents the 21 difference in phase between these two signals (i.e., ~ MR1 a - ~ MR2A).

22 Eq. 2.3: Delta A = (~ TR2A - ~ TR 1 A) + (~ R2E - ~ R 1 E) 23 The component of the DeItaA signal representing the true 24 phase difference (~TR2A - ~TR1A) is a function of the resistivity of the DOCKET NO. 414-4780-US

-22- _ 21543'8 1 formation in the region between the two receiver coils. Let F(rho) represent 2 this component.
3 Eq. 2.4: DeItaA = F(rho) + (~ R2E - ~ R1 E) 4 Similarly, during Sample 8, the ~ MR2B signal and the ~ MR1 B
are simultaneously processed to produce a DeItaB signal that represents the 6 difference in phase between these two signals (i.e., ~ MR2B -~ MR1 B).
7 Eq. 2.5: ~MR1B = ~TR1B + ~R1E
8 Eq. 2.6: ~ MR2B = ~TR2B + ~ R2E
9 Eq. 2.7: DeItaB = (~ TR1 B - ~ TR2B) + (~ R1 E - ~ R2E) The component of the DeItaB signal representing the true 11 phase difference (~TR1B - ~TR2B) is a function of the resistivity of the 12 formation in the region between the two receiver coils; i.e., it equals f(rho).
13 Eq. 2:8 DeItaB = f(rho) + (~ R1 E - ~ R2E) 14 The Delta A signal is recorded so that it can be retrieved and processed with the Delta B signal.
16 By adding Equations 2.7 and 2.8, it follows that:
17 DeItaA + DeItaB = 2 * f(rho) + ~ R2E - ~ R1 E - ~ R2E + ~ R1 E and 18 Eq. 2.9: f(rho) = 1 /2 * (DeItaA + DeItaB) 19 In other words, a computed signal representing the sum of the consecutive samples is a function of formation resistivity, and error 21 components such as ~ R1 E and ~ R2E do not introduce errors into this 22 computed signal.
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2.54378 1 Figures 2A, 2B, 2C, 2D, and 2E depict the electronics carried 2 by measurement sonde 27 in block diagram and electric schematic form.
3 As is shown in Figure 2A, processor 101 directs the operation of drive 4 circuits 105, 107 and receiver circuit 103. Drive circuit 107 operates to energize selectively transmitting antenna T2, while drive circuit 105 operates 6 to energize selectively transmitting antenna T,. Receiver circuit 103 receives 7 the measurements of the electromagnetic field made by receiving antennas 8 R~, R2. Processor 101 supplies data to telemetry system 109, and receives 9 instructions from telemetry system 109. Telemetry system provides the electrical commands to a mud pulse telemetry actuator which is in 11 communication with a fluid column in the wellbore, and which is utilized to 12 impress a coded message in the fluid column.
13 The principle components of the electronics carried by 14 measurements sonde 27, which are depicted in block diagram form in Figure 2A, are depicted in detail in Figures 2B, 2C, 2D, and 2E, with 16 Figures 2B, 2C, and 2D respectively depicting the dual receiver 111, 17 amplitude/phase detector 113, and the local oscillator 115 which comprise 18 the significant operational components of receiver circuit 103, and with 19 Figure 2E depicting drive circuit 107 of Figure 2A, which is identical to drive circuit 105, of Figure 2A.
21 First with reference to Figure 2B, there is depicted dual 22 receiver 111 in simplified electrical schematic form. In broad overview, the 23 signals from receiving antennas R,, R2 are fed to dual receiver 111. The 24 signals are amplified at RF amplifiers 119, 121 and heterodyned with the output of the 1.995 megahertz local oscillator 117, which provides a 26 resultant 5 kilohertz intermediate frequency (I F). The intermediate frequency DOCKET NO. 414-4780-US

21543~~

1 will retain the phase relationship of the two receiver signals by using a 2 common oscillator for mixers 123, 125. The signals are passed to 3 intermediate frequency (IF) amplifiers 127, 129 for further amplification.
The 4 signals are then passed through 500 hertz band pass filters 131, 133 before being fed to the amplitude/phase detector 113 of Figure 2C.
6 Now more particularly, the dual receiver 111 of Figure 2B
7 receives the 2 megahertz signal from receiving antennas R,, R2, which are 8 coupled to the input of RF amplifiers 119, 121. The coupling components 9 are specifically selected to minimize phase shift with temperature. The RF
amplifiers 119, 121 have a gain of l5db and the output is coupled to the 11 balanced mixers 123, 125. The output of the local oscillator is injected to 12 the mixer via a power splitter arrangement to provide equal amplitudes with 13 zero degrees of phase shift to each mixer. The splitter, combined with a 14 6db attenuator, will ensure the proper level and also minimize cross talk between the two mixers. A balanced mixer is used to minimize local 16 oscillator feed-through to the output. The mixer stage has a gain of 20db.
17 The mixer output is fed through five kilohertz low pass filters 135, 137 and 18 coupled to the non-inverting input of IF amplifiers 127, 129, which provides 19 an l8db gain. The output of IF amplifier 127, 129 is fed through 500 hertz band pass filter op amps 131, 133 which provide a 20db gain. This final 21 amplification has a very high Q 500 hertz band pass filter. The passive 22 components of the filter must be matched to ensure minimal phase shift with 23 frequency and temperature. It is important that both filters remain matched.
24 The amplitude/phase detector 113 is depicted in Figure 2C.
The amplitude/phase detector 113 has a separate automatic gain (AGC) DOCKET NO. 414-4780-US

2154~~

1 circuit for each IF signal received, which provides constant amplitudes for 2 the phase detector. The AGC circuit's control voltage to the is proportional 3 to the amplitude of the IF signal providing a DC signal corresponding to the 4 received signal level. The DC level is used by the processor as the amplitude of the received signal level. The output of the phase detector is 6 a pulse which is proportional to the phase shift between the two IF signals.
7 This voltage is integrated and amplified to provide phase outputs of 100°
8 and 20°. The resultant signal is fed to the microprocessor board which 9 initiates a measurement cycle, times the events during that cycle, samples data from the receiver, stores data in memory, and communicates with a 11 tool bus.
12 The automatic gain control circuit has a dual purpose: to 13 detect the amplitude of the incoming signal from the dual receiver 111 of 14 Figure 2B, and to maintain a constant amplitude signal to the phase detector. The automatic gain control contains for each channel an Analog 16 Devices linear divider 139, 141. The output of the linear dividers 139, 141 17 (V~ is equal to the intermediate frequency IF divided by a DC level (V=).
18 The output of each analog divider 139, 141 is connected to low pass filter 19 143,145, with a cut off of 7.5 kilohertz to remove any high frequency noise.
This signal is fed to an RMS-to-DC converter 147, 149. An error amplifier 21 155, 157 compares the output of converters 147, 149 to a reference signal 22 provided by reference signal generators 151, 153, and generates a control 23 voltage dependent upon the difference. To prevent a loop lock-up, negative 24 values resulting from the loss of signal are diode-blocked by diodes 159, 161 which are coupled between error amplfiers 155, 157 and output buffers 26 163, 165. The control voltage is fed to the automatic gain control to 27 maintain a constant output, and to the processor for amplitude information.
DOCKET NO. 414-4780-US

21543'~~

1 The IF signals from the automatic gain control circuit are capacitively 2 coupled to inverting buffers 167, 169 for phase detection, to eliminate any 3 DC offset. Preferably, the IF2 signal is inverted 180°. This allows the output 4 of the phase detector to be in the range of -180° to + 180°, instead of being in the range of 0° to 360°. The signals are then squared-up with a zero 6 crossing detector 171, 173, and passed to a 4013 dual flip-flop set which 7 function as phase detector 175. To accomplish this, the supply voltage flip-8 flop is not grounded. Instead, a positive voltage is powered from a floating 9 supply so that what would normally be considered a low voltage {ground) will be a negative voltage or -180°. Likewise, a high level will be a positive 11 voltage, or + 180°. The clock inputs are used so that the phase detector will 12 not be sensitive to cycle variation in the IF square waves. The phase 13 detector output will be as follows:
14 (1 ) a square wave input from receiver R, intermediate frequency IF, sets pin 1 high;
16 (2) a square wave input from receiver R2 intermediate frequency 17 IF sets pin 13 high;
18 (3) a high on pin 13 resets both flip-flops sending pin 1 low.
19 If the receive signals are in phase, the signals to the flip-flop would have 180° of phase difference due to the inversion of IF2. This would 21 result in the phase detector having a 50% duty cycle on pin 1, switching 22 between negative 4.5 volts and positive 4.5 volts. This would result in zero 23 volts on the low pass filter of integrator amplifier 177. As the phase 24 differential increases, so does the positive pulse width, causing a negative voltage out. Likewise, a negative phase input causes a negative pulse width 26 resulting in a negative DC level from the filter. The gain in the low pass filter 27 is set so that 100° equals 5 volts. This signal is used by the processor for DOCKET NO. 414-4780-US

2.14378 1 the plus or minus 100° input and is also amplified five times by DC
amplifier 2 179, for the 20° of phase difference to equal a 5 volt output.
3 Figure 2D depicts a local oscillator 115 which generates the 4 1.995 megahertz injection for the receiver mixers of Figure 2B. Local oscillator 115 consists of a voltage controlled oscillator, a phase lock loop, 6 and a final amplifier. A 20 kilohertz reference is supplied for the phase lock 7 loop. A voltage controlled crystal (Colpitts) oscillator is utilized to reduce 8 frequency error due to vibration downhole. The voltage controlled oscillator 9 is locked to a reference frequency so the IF will be 5 kilohertz.
A higher frequency crystal (7.982 megahertz) is used to increase the tuning range of 11 the circuit. The oscillator is tuned to the desired center frequency by 12 variable capacitance diodes VC1 and VC2. Control voltage for the diodes 13 is supplied by the phase lock loop as follows. The output of the oscillator 14 is divided by 4 to obtained the desired 1.995 megahertz frequency which is coupled to pin 9 of an MC14569 programmable binary down counter.
To 16 achieve a division ratio of 399, the MC14569 is cascaded with the 17 programmable counter in the MC14568. The remaining counter in the 18 MC14568 is used to divide the 20 kilohertz reference by 4 to provide a 5 19 kilohertz reference for the phase detector. The 5 kilohertz out of the divider (1.995/399) is compared to the 5 kilohertz reference by the MC14568 phase 21 detector. Pulses out of the phase detector on pin 13 represent the 22 frequency error between the divided 1.995 megahertz and the 5 kilohertz 23 reference. These pulses are integrated by capacitor C4 to provide a 24 correction voltage for diodes VC1 and VC2 to bring the oscillator back on frequency. The 1.995 megahertz is buffered by three 4049 inverters which 26 are tied in parallel. A Pi network is used to match the output of the 4049's 27 to 90 Ohms and also attenuate the harmonics of the 1.995 megahertz DOCKET NO. 414-4780-US

_28_ _ 1 square wave. The power output of the local oscillator is approximately 10 2 milliwatts or + 1 Odbm.
3 Figure 2E depicts drive circuit 107 of Figure 2A, which is 4 identical to drive circuit 105 of Figure 2A. Each drive circuit is identical, except that each has a different transmit control voltage. Each transmitter 6 drive circuit consists of a voltage controlled crystal oscillator, a phase lock 7 loop, a final amplifier, and a transmitter on/off control. The 20 kilohertz 8 reference is provided. To ensure a satisfactory lock range, a 8 megahertz 9 voltage controlled crystal oscillator is used in the transmitters. The frequency is adjusted by the variable capacitants of VC1 and VC2. The 11 output is divided by 4 to obtain the desired 2 megahertz final frequency.
12 The two megahertz output signal is coupled to the phase lock loop, which 13 in turn divides the 2 megahertz signal by 100 to obtain the 20 kilohertz 14 reference frequency to compare with the 20 kilohertz from the phase detector. Pulses from the phase detector represent the frequency error. A
16 correction voltage is supplied to VC1 and VC2 to bring the oscillator back ~ 7 on frequency. The 2 megahertz signal is also tied to U2 which controls the 18 output of the transmitter. The output stage is driven into class D
operation 19 by using a square wave input which results in higher amplifier efficiency.
A
low pass filter is used to reduce the harmonics. The cut off for this filter is 21 3 megahertz. The power output of the transmitter is approximately 0.5 watts 22 (27dbm).
23 Figures 3A, 3B, 4A, 4B, 4C, and 4D depict two embodiments 24 of the present invention. Figures 3A and 3B depict measurement tubular 401 and measurement sonde 419 which is adapted to be positioned within 26 the central bore 410 of measurement tubular 401. Measurement tubular DOCKET NO. 414-4780-US

-29- _ 2.5437 1 401 is composed substantially of steel, as are other prior art drill collars;

2 however, measurement tubular 401 includes four regions which include a 3 plurality of axial slots which are disposed circumferentially about 4 measurement tubular 401 and which extend through the width of measurement tubular 401, but which are filled with a poorly-conducting or 6 non-conducting material, such as a KEVLAR material or such as an epoxy 7 or ceramic material. The axial slots which are filled with non-conducting or poorly-conducting material allow for the inward and outward passage of 9 electric and/or magnetic oscillating fields, but which prevent the passage of fluid through measurement tubular 401. As is shown in Figure 3A, upper 11 transmitter region 402 includes the axial slots which allow for the inward and 12 outward passage of electric and/or magnetic oscillating fields.
Likewise, 13 lower transmitter region 407 includes the axial slots which allow for the 14 inward or outward passage of electric and/or magnetic oscillating fields.

Receiver regions 403, 405 are provided in a position intermediate the 16 transmitter regions 402, 407. Receiver regions 403, 405 also include the 17 axial slots filled with poorly-conducting or non-conducting material, which 18 allow for the inward or outward passage of electric and/or magnetic 19 oscillating fields. The regions which contain the axial slots filled with poorly-conducting or non-conducting material are separated and surrounded by 21 solid regions 409, 411, 413, 415, and 417, which do not allow for the 22 passage of electrical and/or oscillating fields, since they are composed of 23 steel which dissipates the electrical and/or magnetic oscillating field by the 24 formation of eddy currents. Measurement sonde 419 is disposed adjacent measurement tubular 401 in the depiction of Figure 3A. In actual use, 26 measurement sonde 419 is disposed within the central bore (which is 27 depicted only in Figure 3B) of measurement tubular 401. Measurement 28 sonde 419 is composed of upper transmitter assembly and pressure DOCKET NO. 414-4780-US

2.54378 1 housing 421 which contains the upper transmitting antenna, receiver 2 assembly and middle pressure housing 425 which contain the receiving 3 antennas, and lower transmitter assembly and pressure housing 429 which 4 contain the lower transmitter. Upper paddle assembly 423 and lower paddle assembly 427 are provided to centralize and position measurement 6 sonde 413 within the central bore of measurement tubular 401. The 7 electrical, electronic, and data processing components which cooperate to 8 allow for the reception and transmission modes of operation are contained 9 within the pressure housings 421, 425, 427. Upper transmitter 431 is disposed on the exterior surface of upper transmitter assembly and 11 pressure housing 421 and is adapted to be aligned with transmitter region 12 402 when measurement sonde 419 is positioned within the central bore of 13 measurement tubular 401. Lower transmitter 437 is carried about the 14 exterior portion of lower transmitter assembly and pressure housing 429 and is adapted in position to be aligned v~i~th transmitter region 407 of 16 measurement tubular 401 when measurement sonde 419 is positioned 17 within the central bore of measurement tubular 401. Receiver antennas 433, 18 435 are carried by receiver assembly and middle pressure housing 425 and 19 adapted in position to align with receiver regions 403, 405 when measurement sonde 419 is positioned within the central bore of 21 measurement tubular 401. The axial slots in measurement tubular 401 22 which are filled with poorly-conducting or non-conducting material allow for 23 the sonde-based measurement of well parameters outside the drillstring 24 which would normally be impeded by the presence of a steel collar. The slots are constructed such that the collar of measurement tubular 401 26 maintains its structural integrity necessary for drilling operations, and drilling 27 fluids are not allowed to flow through the axial slots since the non-DOCKET NO. 414-4780-US

_31 _ 2~~4378 1 conducting or poorly-conducting materials are solid fluid-impermeable 2 materials.
3 Figure 3B is a partial longitudinal section view of measurement 4 sonde 419 disposed within central bore 410 of measurement tubular 401.
As is shown, electronics cavities 412, 414 are provided above and below 6 antenna section 416. Antenna section 416 includes an antenna cavity 418 7 which is filled with fiberglass 420, which surrounds antenna wire 422.
8 Antenna section includes antenna housing sleeve 422 which includes upper 9 lip 424 and lower lip 426 which mate with mandrel 428 and mandrel 430 of measurement sonde 419. As is shown, mud flows between measurement 11 tubular 401 and measurement sonde 419. The electronics contained within 12 electronics cavities 412, 428 are protected from the mud flow.
13 Figure 4A depicts an alternative embodiment of the present 14 invention which is especially useful in slim-hole applications. For purposes of this application, Nslim hole" applications are these which utilize tubulars 16 having and oud outer diameter of six (6) inches or less. Measurement 17 sonde 503 is shown disposed within central bore of measurement tubular 18 501. Preferably, measurement tubular 501 is formed of a reduced radial 19 diameter in the range of three to four inches. Measurement sonde 503 is adapted to engage the central bore of measurement tubular 501, in the 21 areas of the antennas so that the antennas can be as large in diameter as 22 possible and so that the antennas can serve a second purpose and locate 23 the sonde 503 within the bore 501. Measurement sonde soa inch mpg a 24 controller and transmitter electronics subassembly 507, transmitter antenna 509, receiver electronics subassembly 511, receiver antennas 513, 514, 26 transmitter electronics housing 515, and transmitter antenna 517. Figure DOCKET NO. 414-4780-US

2~54~'~$

1 4B is a detail view of the region of transmitter 517. Measurement tubular 2 501 is shown in fragmentary section view. It is equipped with a plurality of 3 axial slots 525 which are filled with poorly-conducting or non-conducting 4 epoxy or KEVLAR material. Transmitter 517 is carried adjacent the axial slots 525. Figure 4C is a cross-section view as seen along section lines C-6 C of Figure 4A. As is shown, a plurality of spokes 527 and axial slots 529 7 are provided. Drilling mud is pumped downward through axial slots 529 in 8 the region of the antennas. This arrangement has several significant 9 advantages over the prior art. Frst, the measurement sonde 503 may be mechanically supported by measurement tubular 501 preventing movement 11 of measurement sonde 503. Second, the close alignment between 12 measurement tubular 501 and measurement sonde 503 prevents the flow 13 of drilling fluids in the region of the transmitting and receiving antennas, thus 14 minimizing the possibility of damage to the antennas. Third, the transmitting and receiving antennas are placed as closely as possible to the axial slots 16 in measurement tubular 501 to increase measurement efficiency and reduce power requirements for the logging tool. The measurement tubular can of 18 course be constructed of a non-conducting or poorly-conducting collar as 19 described earlier instead of a slotted collar.
Figure 4D is a longitudinal section view taken along section 21 line D-D of Figure 4C. As is shown, measurements on 503 is disposed 22 within central bore 510 of measurement tubular 501. Electronic cavities 23 512, 514 are provided for housing the logging tool electronics. Antenna 24 section is provided with a C-shaped sleeve which is filled with fiberglass 516, and which houses antenna wire 518. Flowpaths 520, 522 are provided 26 between C-shaped sleeve and the main body of measurement sonde 503.
27 Drilling mud is routed through flow passages 520, 522 in the region of DOCKET NO. 414-4780-US

2~.543'~~
1 antenna 518. Antenna 518 is placed as closely as possible to measurement 2 tubular 501, which may be formed from a poorly-conducting material, or 3 which may include axial slots which allow for the inward and outward 4 passage of electromagnetic fields.
The embodiments discussed herein have focused on the 6 utilization of resistivity logging tools within measurement tubulars which are 7 formed at least in-part by a poorly-conducting or non-conducting material;
8 however, other types of measurement tools may be formed in and carried 9 on measurement sondes which are placed inside measurement tubulars which are partially formed of non-conducting or poorly-conducting materials, 11 such as dielectric logging tools and other tools which utilize oscillating 12 electric and/or magnetic fields to sense or detect conditions exterior of the 13 logging tool which relate to either the borehole or surrounding formation.
14 One example of such a dielectric logging tool which can be used as is described in the present invention is set forth in U.S. Patent No. 4,107,598 16 to Meador et al. In alternative embodiments, resistivity and dielectric logging 17 tools may be combined in a single sonde to allow for multiple types of 18 measurement to occur simultaneously or successively. The types of 19 information which can be derived from resistivity and dielectric logging tools can be util'~zed to define a plurality of borehole and formation conditions, 21 including a measure of the borehole diameter, a measure of the diameter 22 of invasion of drilling muds, a measure of the formation resistivity, and the 23 utilization of amplitude attenuation and phase shift measurements (which 24 see differing depths of the formation) for the location and detection of bed and boundary changes. An example of the utilization of amplitude and 26 phase data to perform caliper operations in a wellbore, and to detect bed 27 and boundary changes is set forth in U.S. Patent No. 4,899,112 to Clark, DOCKET NO. 414-4780-US

_ z~~437~

1 which is entitled "Well Logging Apparatus For Determining Formation 2 Resistivity At A Shallow Depth And A Deep Depth" .
3 In particular embodiments of the present invention, the 4 measurement sonde may be equipped with a conventional fishing neck disposed at its upper most portion, which allows the measurement sonde 6 to be run into or out of the measurement tubular during drilling operations 7 utilizing a conventional wireline-base retrieval apparatus. Typically, the 8 retrieval apparatus includes a component which is profiled to engage the 9 fishing neck of the measurement sonde. In this way, the measurement sonde may be removed for repair or replacement during drilling operations, 11 without requiring that the entire drillstring be pulled.
12 While the invention has been shown in only several preferred 13 forms, it is not thus limited but is susceptible to various changes and 14 modifications without departing from the spirit thereof.
DOCKET NO. 414-4780-US

Claims

1. ~A measurement-while-drilling (MWD) apparatus for use with a drillstring during drilling operations for obtaining information about a parameter of interest, comprising:

a measurement tubular that includes a central bore that communicates with a central bore of said drillstring;

a measurement sonde located within the measurement tubular for obtaining measurements on the parameter of interest, said measurement sonde including:

at least one transmitter for inducing an electromagnetic signal in the formation through the measurement tubular; and at least one receiver for receiving the induced electromagnetic signal in the formation through the measurement tubular; and a retrieval member on the measurement sonde adapted to facilitate an operation selected from (i) retrieval of the measurement sonde from the measurement tubular during drilling operations, and, (ii) placement of the measurement sonde within the measurement tubular during drilling operations.

2. ~The MWD apparatus of claim 1 wherein the at least one transmitter comprises two transmitters and the at least one receiver comprises two receivers.

3. ~The MWD apparatus of claim 1 wherein the measurement tubular adjacent the at least one transmitter and the at least one receiver is made of a material selected from (i) a poorly conducting material, and (ii) a non-conducting material.

4. ~The MWD apparatus of claim 1 wherein the measurement tubular adjacent the at least one transmitter and the at least one receiver is made of a material comprising carbon fiber.

5. ~The MWD apparatus of claim 1 wherein the measurement tubular comprises a member made of a conducting material and wherein a portion of the member adjacent the at least one transmitter and the at least one receiver is provided with a plurality of axially aligned slots circumferentially disposed around the member, said axially aligned slots being filled with a material selected from (i) a poorly conducting material, and (ii) a non-conducting material.

6. The MWD apparatus of claim 5 further comprising a second member disposed around the first member, said second member being made of a material selected from (i) a poorly conducting material, and (ii) a non-conducting material.

7. The MWD apparatus of any one of claims 1 to 6 wherein the measurement sonde further comprises a data processor for processing said measurements on the parameter of interest.

8. The MWD apparatus of claim 2 wherein the two receivers are disposed between the two transmitters.

9. The MWD apparatus of any one of claims 1 to 8 wherein the measurement sonde is powered by one of (i) a battery, and (ii) a turbine.

10. The MWD apparatus of any one of claims 1 to 9 wherein the parameter of interest is at least one of (i) the resistivity of the formation, and (ii) the dielectric constant of the formation.

11. The MWD apparatus of claim 1 wherein the MWD apparatus is adapted for slimhole operations with the measurement sonde engaging the central bore of the measurement tubular in a portion of the measurement sonde in the vicinity of the at least one transmitter and the at least one receiver (the "antenna sections").

12. The MWD apparatus of claim 11 wherein each antenna section further comprises:
a plurality of axial slots for passage of drilling mud;
a C-shaped sleeve for isolating an antenna in the antenna section from the drilling mud and urging the antenna against the measurement tubular; and a plurality of spokes defining the axial slots therebetween and for supporting the C-shaped sleeve.

13. The MWD apparatus of claim 12 wherein each antenna section further comprises at least one electronic cavity for housing electronic equipment associated with the antenna.

14. A method of obtaining measurements relating to a parameter of interest of a formation surrounding a borehole drilled by a drillbit conveyed on a drilling tubular, the method comprising:
detachably coupling a measurement sonde within a measurement tubular having a central bore in communication with a central bore of said drilling tubular;
activating a transmitter on the measurement sonde thereby inducing an electromagnetic signal in the formation through the measurement tubular; and detecting with a receiver on the measurement sonde a detected signal indicative of the induced electromagnetic signal and the parameter of interest.

15. The method of claim 14 wherein the measurement tubular adjacent the transmitter and the receiver is made of a material selected from (i) a poorly conducting material, and (ii) a non-conducting material.

16. The method of claim 14 wherein the measurement tubular comprises a member made of a conducting material and wherein the portion of the member adjacent the transmitter and the receiver is provided with a plurality of axially aligned slots circumferentially disposed around the member, said axially aligned slots being filled with a material selected from (i) a poorly conducting material, and (ii) a non-conducting material.

17. The method of any one of claims 14 to 16 further comprising processing the detected signal with a data processor on the measurement sonde to give the parameter of interest.