DE1623118C3 - Geophysical logging direction - Google Patents

Description

The invention relates to a geophysical logging device with the features specified in the preamble of claim 1.

Such a device is known from US-PS 31 47 429 and CA-PS 7 21 843. The well-known The facility is a so-called induction log, i. In other words, it works with transmission frequencies at which you can in. the earth formations built up field can be seen as quasistatic, in the sense that the speed of propagation of the electrical waves in the formations is negligible. One can therefore assume that with the known Induction logs even if they have several receiving antennas (DE-AS 12 00 971, DE-PS 8 98 642), the transit times are irrelevant.

If you choose higher frequencies, for example with the induction log, this occurs in all high-frequency applications common skin effect, d. H. the current displacement due to the interaction of the currents with the magnetic field generated by them. The induction log measuring units must then, for example, with Using tables, corrected accordingly.

ίο A characteristic value of the earth formations, which is for a Borehole measurement is interesting, is the conductivity of the so-called "invasion zone" next to the borehole, d. H. in the area of the borehole wall in which the drilling fluid displaces the formation fluid Has. So far it has been customary to carry out this measurement using an electrode probe, in which the Electrode-bearing pads are in direct contact with the wall of the borehole. This is not only a hindrance fast special trips in the borehole, but the measurement is also influenced by the currents that be induced by an induction log carried along at the same time.

The invention has for its object to ζ a measuring device of the type mentioned? '

2 ^r > at which the conductivity of the invasion layer - if not only this - can be measured using a kind of induction logging method. The solution to this problem results from the characterizing features of claim 1,

3 () while the dependent claim defines an embodiment, the meaning of which in connection with the Embodiment is explained in more detail.

It follows from the main claim that considerably higher frequencies are used here than with known induction logs, so that phase and / or amplitude gradients can be recorded; It should be noted that field gradients are known per se, for example from US Pat. No. 2,996,663.
The following explanation of exemplary embodiments with reference to drawings and diagrams provides a complete picture of the invention and the physical relationships. It shows
F i g. 1 a borehole sunk in the earth with a graphic representation of lines of the same , ■ phase, which have been generated by electromagnetic waves during V-propagation through the borehole and the surrounding earth formations,
2 shows a so-called "phasor" or phase diagram of the magnitude and phase of the electromagnetic fields as they propagate through the ground, FIG. 3 shows a coil arrangement in a borehole together with a schematic representation of the electrical circuit technology used in connection with the coil arrangement for the investigation of earth formations according to the invention,

F i g. 4 is a graphical representation of certain factors involved in determining the so-called "Spacings" or distances between the transmitter and receiver of the arrangement as well as the operating frequency of the device according to FIG. 4 must be observed should,

F i g. 5 an elec-

b5 electrode arrangement or group together with schematic Representations of electrical circuit arrangements that are used according to the invention can come, and

F i g. 6 shows an apparatus in an arrangement according to another feature of the invention.

The relationship for the magnetic field strength H ₁ at a distance ζ from a transmitter or transmitter. <Ann for large values of ζ derived from Vlaxwell's equation can be expressed as:

/ Z ₂ = H ₀ e

ei e equals 2.718 the base of the natural logaithm, H _{0 means} the magnetic field strength at the iender or transmitter,

ind is the so-called "skin" depth, which can be defined as

α μ η

/ obei in = 2 .τ / is the angular frequency of the transmitter or other signal, // means the magnetic permeability of the formation in question and; η is generally to be regarded as a constant and is the conductivity or conductivity of the formation in question .

Equation (I) can be rewritten in the case of the electric field strength by sub-substituting E for H. Equation (1) expresses that the propagation of the electromagnetic field is weakened and phase shifted with an increase: cs distance value z, i.e. with propagation of the electromagnetic energy through the earth formations . The degree of phase shift is expressed by the term - ~ of equation (I) and the degree of attenuation is expressed in

em term - ^ - of equation (1). Thus the term is

- (1 + j) defined as the propagation constant, the term - ^ - as the attenuation constant and the

erm —7 "- as the phase constant. The equation (1) > So it says that an electromagnetic wave is weakened by a factor of - per "skin" depth Λ

earth and is phase-shifted by a radian ir every "skin" depth <) of its path.

As stated above, in the conventional induction log or electrode log systems, the working or operating frequency is kept sufficiently low so that the static or quasi-static electricity theory can be used. This means that the field build-up at one point in the earth formation by the transmitter or transmitter has a negligible effect on the earth at other points in the earth formation. In order to apply this static or quasi-static field theory, however, the working frequency of the operating frequency must be sufficiently low so that the "skin" depth in the formations is significantly greater than the spacing or the distance between the transmitter or transmitter and the receivers . This can be seen better from equation (1), where if the "skin" depth Λ is substantially greater than ζ, H ₂ is substantially equal to Zf ₀ for any small value of z.

In contrast to the induction log or

The static or quasi-static field theory applied to the electrode log method has the The present invention is based on an increase in the working or operating frequency of the Borehole investigation system to a point where the "skin" depth is on the order of the Spacings or distance between the transmitter or transmitter and the receiver is and usually

r> is practically smaller than this spacing. Due to the significant frequency increase compared to the working or operating frequency with the usual Induction logging or electrode logging methods can be used in conjunction with these well known borehole logging methods no longer used static or quasi-static field theory come. Instead, the electromagnetic field builds up at one point in the earth formations a significant impact on the field at others

> ^r > places in the formation, which gives rise to an electromagnetic sky wave. This can be seen from equation (1), where if the "skin" depth ί> is of the order of magnitude or less than the distance ζ , the magnetic field strength changes

jo as an exponential function of distance or des Distance z.

In Fig. 1, there are shown lines of same phase 10 generated by plane electromagnetic waves propagating through a formation 11 and a borehole 12. A very similar representation would result based on the field weakening or attenuation. In the case of the FIG. 1, the resistance R _{f of} the formation 11 is greater than the resistance /? ,,, of the drilling fluid. This gives a rise in the lines of same phase in the mud column when examining those in the formation. These lines of the same phase are shown at a mutual distance of a »skin« depth. In order to generate such plane waves, the transmitter or transmitter must be located at a considerable distance from the receivers R ₁ and R ₂ . The transmitter or transmitter and the receiver are used to transmit or receive electromagnetic energy and can therefore be referred to as antennas. These transmitting and receiving antennas can either be formed by electrodes or coils.

Since the flushing column 12 is smaller in size than the rest of the earth formation, it will essentially

Vi borrowed all the energy sent into the earth propagate through the formation instead of through the mud column, which is why spacing or the distance between the lines of the same phase in the flushing column is equal to the spacing or

ho stood between the lines of the same phase in the formation will be. Therefore, by placing a pair of receivers K ₁ and R ₂ at a mutual distance I / in the borehole 12 and by comparing the phase of the two Etmp-

h ^r > R ₁ and R ₂ received signals catch the "skin" depth in the hatched area of the earth formation between the lines of the same phase, which are in connection with the receivers R ₁ and R ₂ , measured

will. The ratio for this phase difference I Φ can be represented as:

Λ Φ =

Λ =

Δι

ι) '

/ 10

(3)

(4)

It can thus be seen from equation (4) that the extent of the phase difference actually forms a measure for the "skin" depth δ _{F of} the formation. This phase difference can also be referred to as a phase gradient. Since the "skin" depth is functionally related to the conductivity or conductivity of the formations by equation (2), a measure of the "skin" depth <) _F also creates a measure for the conductivity or conductivity a _{F of} the formation. Thus, by inserting equation (2) into equation (4) and solving for the conductivity or conductivity a _F :

a _F = K. \ Φ ² ,
2

(5)

K -

(ι) μ. I / "

In addition to the phase difference between the two receivers K ₁ and K ₂ , the amplitude gradient could also be measured in order to create a measure of the “skin” temperature. This finds its basis in the previously mentioned theory that the electromagnetic wave by one factor - for each

"Skin" -low of its way is weakened or dampened. The part of the formation examined is that in FIG. 1 section shown hatched in the same way as for the case of the phase measurement, since the lines of identical phases of FIG. 1 can also be viewed as lines of equal amplitude. Since it may be desirable to measure the formation's attenuation or attenuation constant, as noted above, the amplitude measurement should be normalized to eliminate absolute amplitude as a factor in the measurements. This can be achieved by taking the ratio of the amplitudes picked up by the two receivers. The relationship for this amplitude ratio A ₂ / A _x can be written as:

A ₁

.— ■ η

(6)

such things as the geometry of the receiver are taken into account.

In Fig. 2 of the drawing shows a so-called "phasor" diagram or phase diagram of the magnetic field strength as the electromagnetic waves propagate or propagate through the formation. The field strength vector Ji ₀ represents the intensity at a point above the receivers R ₁ and R ₂ (not on the transmitter or transmitter). With spread

ίο or propagation of the electromagnetic wave through the earth the field strength is increased by a

Factor - weakened or subdued and around one

Radians out of phase with each skin depth of its path, as explained above. The field strength vectors H _{R {} and HR ₂ reflect the intensities at the receivers K ₁ and K ₂ . A signal is generated in the receivers K ₁ and K _{2 which} is proportional to these field strength vectors Hr ₁ and H _R2. By

equal to the phase or amplitude of these two field strength _vectors H Rl and H _R2 , in order to create a measure of the phase or amplitude gradient of the magnetic field in the vicinity of the receiver, the phase or attenuation constant of the propagating or propagating electromagnetic energy in the vicinity of the receiver K. ₁ and K ₂ can be determined. This then gives a measure of the skin depth a _{F of} the formation. Then the conductivity (T _{f of} the formation can be determined from equation (5)

jo or (7).

The phase or attenuation constant measured by the receivers K ₁ and K ₂ is, because of the differential receiver arrangement, as set out above, only by the in FIG. 1 hatched part of the formation affected. Thus, the influence of the formations above this dashed formation part on the propagating electromagnetic wave is effectively eliminated by the differential receiver arrangement. In other words, the first or upper receiver K ₁ acts as a reference point or reference variable for the second or lower receiver K ₂ . This can be seen more clearly from FIG. 2, if the vector H _{R is taken} as the reference phase and amplitude for the second vector H _R. It is thus understood that any influence on the electromagnetic wave before it reaches the first or upper receiver K _{1 has} essentially no influence on the measurement at the two receivers K ₁ and K ₂ .

To give an example of this, it is assumed that the receivers K ₁ and K _{2 are} a "skin" depth 6 _F apart from one another, as in FIG. 1 shown.

In this case the vector H _{Ä is} equal to - H _R. and

out of phase by one radian from H _R. The ratio H _R JH _R is then the same

where A _{2 is} the amplitude of the signal received _{by receiver K 2} and A _{1 is} the amplitude of the _{signal received by receiver K 1} (ie, the received voltage). Solving equation (6) for the conductivity or conductivity a _F results in:

ω μ (Δ I) ²

In ²

(7)

where K _{1 is} a proportionality constant in which what indicates that the receivers K ₁ - K _{2 are} a "skin" depth d _F apart. It is assumed that a formation above the receiver group or arrangement K ₁ - K _{2 has} a higher resistance than originally assumed for the diagram according to FIG

Reld strength vectors take the form represented by the field strength vectors Hr _x and Hr _2. The vector Hr ₂ is then equal to - H _R] . Thus are the amplitude ratio

H _'R

2 _

H _'R

ι _

and the phase difference is still one radian. The differential receiver arrangement thus still indicates that the measured "skin" depth <) _{F is} equal to the spacing or distance / I / between the receivers R ₁ - K ₂ .

The above investigation is based on the assumption that the formations are homogeneous and that the transmitter-receiver distance is large enough that the electromagnetic waves can be viewed as plane waves. This assumption has greatly simplified the explanation. In practice, however, the / M measuring formations are not homogeneous, and if the transmitter-receiver distance were too great, it is clear that, due to the high attenuation or attenuation of the electromagnetic waves in the formations, only very little energy remains at all which ones would have to be measured on the receivers. If desired, an in-phase representation of the electromagnetic energy for this inhomogeneous case could be made, whereby it could then be determined that the radial investigation of the formations can be selected in a desired manner by suitable choice of the working or operating frequency and the transmitter-receiver frequency. Spacing or spacings. For example, it may be desirable to examine the so-called "invasion zone" or the "invasion-free" or pure formation zone. This "invasion-free" or clean zone is the zone into which the drilling fluid has not penetrated and which is thus the most distant zone from the borehole.

Before discussing how the operating frequency and the transmitter-receiver spacings can be selected, an apparatus or device for measuring the phase or attenuation constant in accordance with the theory discussed above will first be described. Let us refer to FIG. 3, where a borehole logging device 18 is shown, which is suspended at the end of an armored multi-conductor cable 20 in a borehole 19 for the investigation of earth information 21. The cable 20 is lowered or pulled by a drum winch, not shown. The borehole logging device 18 comprises a coil arrangement with a transmitter or transmitter coil T and two receiver coils R ₄ and R ₅ . The center point between the receiving coils R ₄ and R ₅ is at a distance L ₁ from the transmitting coil T, and the two receiving coils R ₄ and K ₅ are at a distance AL from one another. The inspection tool 18 further includes a liquid-tight electronic cartridge 22 which contains the electrical circuitry submerged in the wellbore. This electronic circuit arrangement is shown in the box 22 shown in dashed lines to the side of the borehole.

In this electronic circuit arrangement, a variable-frequency oscillator 23 excites the transmitter or transmitter T, which emits electromagnetic energy for propagation through the formations. A voltage is induced at the receivers R ₄ and K _{5 which} is proportional to the energy of this electromagnetic space wave at the receivers in the manner discussed above. The receivers K ₄ and K ₅ are connected to a pair of electrically matched amplifiers 24 and 25, the of the

ίο amplify received signals _{K 4} and K _5. These amplifiers 24 and 25 are adapted in such a way that any drift in the amplifiers, for example due to a change in temperature, has exactly the same influence on both amplifiers and is therefore essentially eliminated due to the differential receiver arrangement. The signals coming from the amplifiers 24 and 25 are applied to a phase or amplitude comparator or comparator 26. Whether the circuit element 26 is a phase comparator or an amplitude comparator depends on whether the phase or attenuation constant is measured according to equation (4) or (6) to determine the skin depth δ and thus the conductivity or conductivity σ.

The phase or amplitude signal given by the comparator 26 is then transmitted via a two-pole or double switch 34 to a transmission circuit 27 for transmission to the earth's surface passed through a pair of conductors 28, which in reality through the armored multi-conductor cable 20 to the surface of the earth leads. If the phase or amplitude comparator 26 is of such a type that it does the applied input signals into a compared phase or amplitude proportional direct current signal converts, the transfer circuit 27 can only be formed by an amplifier. Sets the Phase or amplitude comparator 26 does not convert the compared signals into a direct current signal, see above the transfer circuit 27 can be a suitable circuit be for the transmission of high frequency signals via the pair of conductors 28 to the earth's surface, z. B. a mixer circuit to set the transmission frequency to one within the transmission possibilities of the Reduce the cable's lying frequency.

At the earth's surface, the pair of conductors 28 feeds the measured phase or attenuation signal to suitable signal processing circuits 29 which, for. B. provide appropriate gain and gain control. If the signal transmitted through the cable 28 is an alternating current signal instead of a changing direct current signal, the signal processing circuit 29 could additionally effect a suitable rectification of the transmitted signals. Since the signals transmitted to the surface via the cable conductors 28 are representative of either the phase or the attenuation constant of the propagation constant in accordance with either equation (4) or (6) in this embodiment shown in FIG. 3, the signals from the signal processing circuits 29 are given to a suitable conductivity or conductivity computer 30 in order to calculate the conductivity σ measured by the borehole logging device according to one of the equations (5) or (7). The resulting values for the conductivity σ are recorded by a recording device 31 which is driven as a function of the borehole depth via a shaft 32 connected to a wheel 33.

809 531 / H

The rotating wheel 33 is in contact with the cable 20 and is rotated in accordance with its up and down movement, so that the recording strip of the recording device 31 is moved as a function of the borehole depth. In this way, the conductivity or conductivity σ is recorded by the recording device 31 as a function of the borehole depth. If desired, with a suitable division of the lines on the recording strip of the recording device 31, the conductivity computer 30 can be omitted, or, in other words, the phase or amplitude function Λ or AgJA _{Ra can be recorded} directly on the recording device 31.

In the operating state, the transmitter or transmitter T works at a constant frequency, sends a constant electromagnetic energy into the surrounding media, which electromagnetic energy is continuously measured by the differential receiver arrangement, and a phase or damping signal is transmitted to the earth's surface in order to use of the recording device 31 to hold conductivity displays. This differential receiver arrangement then comprises the receivers R ₄ and R ₅ as well as the amplifiers 24 and 25 and the comparator 26.

For the sake of simplicity of explanation it is assumed that the emitted electromagnetic energy corresponds to the one shown in FIG. 3 takes the path shown by the arrows. The energy received by the receiver R ₄ takes the path TABR ₄ . and the energy received by the receiver R _{5 takes} the path T-A-BCR ₅ . It can be seen that the energy taking the path T-A-B is common to both receivers R ₄ and R ₅ . It can also be assumed that the paths BR ₄ and CR ₅ are essentially the same with regard to the attenuation and the phase shift of the propagating electromagnetic energy. Thus, due to the differential receiver arrangement, the common path T-A-B and the similar paths BR ₄ and CR ₅ are practically omitted, and the receivers R ₄ and R ₅ actually only measure the path BC.

As in connection with Fig. 1, however, due to the borehole influences, the vertical measuring interval will be more like that shown by the path DE . In any case, it can be said that an earth formation area is examined which has approximately the same extent in the vertical direction as the receiver distance Δ L and which is by and large opposite the receiver arrangement R ₄ -R ₅ . It will be understood that this illustration of the flow of energy is overly simplified in order to provide a simple explanation of the measurement technique.

With regard to the choice of frequency and the transmitter-receiver distances for the investigation of the invasion zone, it is necessary _{to choose the appropriate working frequency / and the distance L (r} between transmitter and receiver so that the invasion zone measurements are not affected by the flushing resistance R _n , or the resistance R, of the invasion-free zone. The receivers should be placed far enough away from the transmitter, in such a way that the electromagnetic wave propagating through the mud column is no longer a factor, and on the other hand close enough to the transmitters that the electromagnetic wave is not propagated into the invasion-free zone to such an extent that it affects the conductivity measurements of the invasion zone.

Yet another factor should be considered in determining

the distance L, _r between transmitter and receiver must be taken into account, namely the fact that the conductivity of the various formation zones has an influence on the "skin" depth of each particular zone [see equation (1)]. Therefore, the expected maximum and minimum "skin" depth

lü values when determining the sender-receiver distances must be taken into account.

To ensure that most of the energy received by the receivers is affected by the invasion zone and not by the mud column in the borehole or by the invasion-free zone, the transmitter-receiver distance should be at least one and a half times the borehole diameter. In addition, the transmitter-receiver distance should be at least half the expected maximum skin depth of the invasion _zone fi x0 and less than approximately fifteen times the expected minimum value of ^ ₀ . This last factor, ie L _tr < (^ ₀ ("" · "), also ensures that the receivers are reached by energy ν sufficient for a measurement.

The distance .1 L between the receivers R ₄ and R ₅ can be within certain limits. These limits relate to the measurement of the phase constant of the propagation constant, ie the phase difference between the two receivers. To understand this better, refer to FIG. 2 came back. If there is a phase difference between the two vectors H _R1 and H _R2 of more than 180 °, it becomes difficult to measure this phase difference.

It can thus be said that the receiver distance. I L should be less than three times the minimum expected "skin" depth of the invasion zone, ie

, 1 L <3 S _x0

(min)

In practice, this receiver distance AL is determined in accordance with the desired vertical resolution of the measuring system, since Δ L determines the vertical resolution, as in FIG. 1 shown.

Obviously the receiver distance is not too important when measuring the attenuation constant, except for vertical resolution.

One way to take a look in a simplified way

to throw at this invasion zone investigation,

thus the electromagnetic field that excites the receiver is concentrated in one area imagine the one just beyond the last transition area between the two zones with different Conductivities lies within a cylinder with a radius equal to the distance between the sender and the receivers. Thus, the investigation of the invasion zone would be this door the last transition area will be the area between the borehole and the invasion zone. Such a transition area can also be viewed as the wall of a wave guide that the transmitter is using connects the recipients and has considerable losses, so that the is immediately within the Wave guidance propagating waves considerably

it will be dampened.

It seems desirable at this point to show the influence of the borehole on the invasion zone investigation. F i g. 4 shows a graph

position of the calculated percentage error Y as a function of

can to keep the "skin" depth t) _x0 constant and thus the function

for different values of

In Fig. 4 it can be seen that the error Y becomes quite significant for certain contrasts of

if

Rm
Ur

is equal to 0.5. If, of course, the contrast between the resistance R _{x0 of} the invasion zone and the resistance R _n , of the flushing were sufficiently large, an examination system in which the transmitter-receiver distance L, _r is half the "skin" depth <) _x0 in the invasion zone will provide sufficiently accurate results. However, since this contrast of resistance cannot always be so high, the ratio may be more desirable

> xO

to be equal to 1, 2 or more in order to keep the percentage error Y low. Thereby reasonably accurate results can be obtained for essentially any contrast or contrast of

xO

It should also be noted here that R _n , can be readily determined at the wellbore, and, since the range of invasion zone resistances R x0 is generally known for any given geographic region, the _{operating frequency can be estimated from} Hand adjusted to keep a ratio of also constant. This is illustrated by switching the two-pole double switch 34 to FIG

ίο Position where the signal from the phase or amplitude comparator 26 to an input of a relatively powerful high power amplifier 39, i.e. H. one High gain amplifier. For the purposes at hand, it is

i ¹ ) add that the output of comparator 26 is a varying direct current signal. A signal of constant voltage proportional to the desired value of the “skin” depth <) _{x0 of} the invasion zone is fed to the other input of the amplifier 39. The amplifier 39 then sends a direct current control signal to the variable-frequency oscillator 23 proportional to the difference between the desired “skin” depth value fi _x0 (desired) and the measured “skin” depth value Λ _χ0 (measured).

During operation, this negative feedback _{arrangement has the effect of keeping} the measured skin depth value <) x0 essentially the same as the desired skin depth _value Λ χ0 by changing the frequency of the variable-frequency oscillator 23. If D _x0 (measured)

ίο equals ^ _x0 (desired), the output signal from amplifier 39 thus has an amplitude of zero. However, if <) x0 (measured) should deviate from the desired value, the amplifier 39 _{supplies the variable-frequency oscillator 23 with a sufficient output}

j5 Signal amplitude increases in order to always bring about the desired condition, ie <) _χ0 (measured) essentially equal to ö _x0 (desired).

In order to obtain a measure of the conductivity σ _χ0 , it is now only necessary to know the size of the control signal given to the variable-frequency oscillator 23. This control signal is of course proportional to the frequency of the variable frequency oscillator 23 since it controls the frequency of the oscillator. Since Ά _{χ0 is} held constant in this case, it is now clear that equation (1) can be rewritten as

2 K,

'xO

sufficiently large to reduce the borehole influence error Y to acceptable values. This is done as shown in FIG. 4 is shown schematically in that the oscillator 23, which excites the transmitter T , is designed as a variable-frequency oscillator.

It should also be noted here that, instead of setting the frequency of the variable-frequency oscillator 23 by hand before lowering the examination device into the borehole, a feedback control loop that is to be lowered into the borehole is used (ü μ

where K ₂ is <5 _x0 (desired). Thus, knowing ω = 2 π j, the conductivity can be found. In order to give a signal representative of the conductivity or conductivity to the earth's surface, the control signal from the amplifier 39 is applied to the transmission circuit 27 via the switch 34 instead of the output signal from the comparator 26.
As for the study of the invasion-free zone

bo, the procedure described for the investigation of the invasion zone can also be used for the invasion-free zone. However, the skin depth <\ of the invasion-free zone must be observed instead of the skin depth δ _χ0 of the invasion zone. The radical distance between the central axis of the borehole and the boundary between the invasion zone and the invasion-free zone must also be taken into account instead of the borehole diameter.

Thus, in the case of this invasion-free zone, the transmitter-receiver distance should be at least about three meters and at least half as large as the expected maximum skin depth ti, _{{max) of} the invasion-free zone in order to obtain an accurate measurement of the skin depth l ), the invasion-free zone and thus the conductivity a, without influencing the invasion zone. The transmitter-receiver distance L, _r should also be short enough so that the receivers have enough energy to measure. Since the transmitter-receiver distance L, _r for the investigation of the invasion-free zone is greater than for the invasion zone, the frequency for this invasion-free zone will most likely be lower in order to satisfy the procedural steps suggested above. The factors determining the recipient distance .1 L are the same as those set out in connection with the investigation of the invasion zone. By following the arrangement rule suggested above, at least one factor of the propagation constant can be measured in order to obtain a measure for the skin depth I), the invasion-free zone and thus its conductivity or electrical conductivity a .

Those to the transmitter and the receiver door the investigation Electronic equipment connected to the invasion-free zone may be essentially the same be like the one in Fig. 3 device for the investigation of the invasion zone with the exception, that the parameters of the circuits are slightly different due to the reduced frequency. In However, in block diagram form, the device for examining the invasion-free zone may be identical to the device for the invasion zone investigation.

It may be desirable to provide an apparatus for examining the invasive zones and the non-invasive zones during the same survey run in the borehole. This can be achieved by using two separate working frequencies and two separate antenna-receiver arrangements. The antenna arrangements for the investigation of the invasion-free zone will then be further away from the transmitter than that for the investigation of the invasion zone. This is in FIG. 3 illustrated by arranging a pair of receivers R ₆ and R ₁ at a sufficient distance below the receivers R ₄ and -R ₅ . A separate oscillator 23a is connected to the transmitting antenna T for this examination of the invasion-free zone. The receivers R ₆ and R ₁ are connected to an electronic circuit 38 consisting of amplifiers, a comparator or comparator and a transmission circuit similar to the amplifiers 24 and 25, the phase or amplitude comparator 26 and the transmission circuit 27, as already discussed above are. Both the oscillator 23 a and the circuit 38 are shown in dashed lines in order to take into account the fact that this double examination arrangement is an alternative embodiment. The signal coming from the electronic circuit 38 is representative of the resistance or the electrical conductivity σ of the invasion-free zone, while the signal given by the transmitter circuit 27 to the earth's surface is representative of the conductivity σ _χ0 or the resistance R _{x0 of} the invasion zone, as discussed above . If desired, a variable frequency arrangement can be provided for the examination of the invasion-free zone in the same way as for the examination of the invasion zone. This is shown in FIG. 3 through the conductor 38a to be provided with the note “control frequency of the variable frequency oscillator 23a”, which is connected to an amplifier similar to the. Amplifier 39 can be connected.

F i g. 5 of the drawing shows another embodiment of the invention for the investigation of underground earth formations by the application of propagating electromagnetic waves. The device according to FIG. 5 shows an electrode arrangement arranged in a borehole 19. The mechanical parts of the well logging tool are omitted from Figure 5 of the drawings for brevity of illustration and description, but these details are well known in the art of logging. The electrodes according to FIG. 5 comprise a current emitting electrode A, a current return electrode B arranged very close to it, and two measuring electrodes M _{1 and JW 2} arranged at a desired distance below the current emitting and returning electrode. The current sending and returning electrodes A and B can be regarded as an electric dipole.

Thus, the combination of the current sending and returning electrodes AB can be viewed as the transmitter for the emission of electromagnetic energy into the surrounding earth formations.

The measuring electrodes M _{1 and M 2} respond to the electric field part of the emitted electromagnetic energy in almost the same way as the receiver coil according to FIG. 3 responded to the magnetic part of the propagating electromagnetic energy. Thus, one can view the measuring electrodes M ₁ and M ₂ as a receiver, and the distance between the transmitting electrodes and the receiving electrodes AB M ₁ -M ₂ is thus the so-called transmitter-receiver spacing or the transmitter-receiver spacing L, _r. The theory discussed above fits the electrode arrangement of FIG. 1 equally well. 5 as for the coil arrangement according to FIG. 3.

The voltage picked up by the measuring electrodes _{M 1 and M 2} , which is proportional to the electric field portion of the electromagnetic wave propagating at the receivers, is applied to a pair of electrically matched amplifiers 40 and 41. The output signals of the amplifiers 40 and 41 are also fed to a phase comparator or comparator 38 as well as to an amplitude comparator 39. Since the phase difference or amplitude ratio of the signals applied to amplifiers 40 and 41 is measured, it will be appreciated that the ground connection for the downhole electronic circuitry is shown in FIG. 5 can be made to the enveloping cartridge, as shown. This means that any error in this grounded reference potential will have very little, if any, effect on the measurements.
As regards the phase comparator 38, the output signals from the amplifiers 40 and 41 are each applied to a pair of voltage limiters 42 and 43 which serve to adjust the voltage level of the signals coming from the amplifiers 40 and 41 at a desired constant voltage keep. The output signals from the voltage limiters 42 and 43 are fed to a phase-sensitive detector 44, the signal from one of the voltage limiters as the reference phase

works for the other signal. Since the voltage limiter 42 and 43 Ampliludcnändcrungen removed from the current supplied to the phase-sensitive detector 44 signals to the phase sensitive detector 44 performs a changing Glcichstrom output signal to the surface that is proportional to the phase difference I Φ between the two of the measuring electrodes Ai ₁ and M ₂ recorded signals.

In the amplitude comparator 39, the output signals from the amplifiers 40 and 41 are passed through a pair of rectifiers 45 and 46 and then fed as rectified output signals to a ratio circuit 47 which takes the ratio of the two input signals. The output signal of the ratio circuit 47 resulting therefrom is thus a changing direct current signal proportional to the ratio V _M JV _M. Both the amplitude and phase signals are carried to the surface of the earth to provide indications of the electrical conductivity of the information surrounding the probe assembly. The comparator 26 of FIG. 3 can be constructed like the phase or amplitude comparator 38 or 39 of FIG. 5.

Another thing must be observed there, namely the influence of the reflected waves on the measurements. Reflected waves are at boundaries such as B. Layer surface boundaries, reflected electromagnetic waves. This problem of the reflected waves causes particular difficulties in connection with the investigation of the invasion-free zone because of the longer scanner-receiver spacing or distance L _lr in this case, which thus gives rise to too long measured depth distances which can be erroneous. Difficulties in terms of reflected waves can also arise when investigating the invasion zone. This reflected wave problem is shown in FIG. 1, which shows an incident electromagnetic wave H, propagating towards a grinding _{surface 13, and a reflected wave J / r} propagating away from the layer surface 13 again.

If the formations are homogeneous and assuming that the antennas are coils which thus generate magnetic fields, the relationship for the magnetic field strength H. of plane waves at a given point in the formation can be expressed as:

where a =

1 ± L

is and

(8th)

Hi - the incident field,
H _r = the reflected field and

z '= the distance from the layer surface plane 13 means.

It has been found that when the second difference in magnetic field is measured, the approaches the second derivative of the field so that the measurement is then essentially insensitive compared to the reflection shaft component of sliding

chung (8) is. The second derivative of equation (8) results in:

U ² IL H. lim άζ ² ~ ~ f IZ .-> =

-2Η.

τ- IZ.) + 1 Η {;

(1L) ²

(9)

The term to the right of the last equal sign of equation (9) is only the approximate definition of a second derivative. From equation (9) it can be seen that the reflection wave component of the magnetic field at a given location can be neglected by placing two receivers with values of +1 each at a distance 1 L from the main central receiver with a value of -2 µm To create a good measure of this second difference, the spacing or the distance I L between the receivers should be relatively short in relation to a skin depth. The relationship for the conductivity or electrical conductivity σ is derived from equation (9):

-2H ₁ + // (ζ- id +

While equations (9) and (10) hold for a homogeneous medium and the case of plane waves, the field equations for spherical waves and inhomogeneous media can be written in the same way and the second derivative of such an equation can be used to determine the spacings or distances and values of the various recipients are used. The same consideration also applies to the electric field quantities, d. H. the electrodes.

In Fig. 6, there is shown an apparatus constructed in accordance with the relationships of equations (9) and (10). This apparatus comprises a coil arrangement having coils R ₉ , R _{10 and R n} in a borehole 50, which are arranged according to equation (9). The receivers R ₉ and /? N are arranged on opposite sides of the central coil Rk, which has twice as many turns as the coils R ₉ and R ₁₁ , in order to create the weighting factors 2, 1 and 1. The coils R ₉ and R _u are connected in series and the central coil R _{10 is arranged} in series with the coils R ₉ and Rn. Thus, each of the coils R ₉ and R _{11 has} a value of +1 and the coil Rj _{0 has} a value of -2 to satisfy equation (9). The series-sonicated coil arrangement is connected to an amplifier and rectifier 51 which gives a signal proportional to the counter of equation (10). In order to obtain a signal proportional to the denominator of equation (10), the _{voltage induced in the middle or central receiver coil R 10} , which is proportional to 2 H ₁ , is applied to an amplifier and rectifier 52 which, for an output voltage proportional to 2 H. cares. This output signal is

809 531/14

then through an evaluation network 53 with a constant factor

weighted to create a signal proportional to the denominator term of equation (10). The outputs from the amplifier and rectifier circuit 51 and the evaluation network 53 are fed to a ratio circuit 54 which gives an output signal to the earth's surface proportional to the conductivity or electrical conductivity n according to equation (10). If desired, instead of connecting the evaluated coils R ₉₇ R ₁₀ UiId R _{n in series,} this evaluation can be achieved electronically by an evaluation network. If the antennas are electrodes, the assessment can also be achieved with suitable electronic networks.

As can thus be seen, the present invention teaches one over the known prior art Technique completely different technique for studying earth formations. While the Present invention electromagnetic energy for the study of underground earth formations used, it is completely different from the earlier static or quasi-static electromagnetic Examination techniques, insofar as the working frequency is increased up to one Point where the static and quasi-static field theory according to the previously known induction log and Elck-Irodenlog investigation systems can no longer be used. Instead, the wavelength is and -> the skin depth of the frequency decreases up to a point, where dynamic field theory is applied and the electromagnetic wave passes through the formations propagated. Then by measuring one or both propagation constants,

ίο d. H. the attenuation constant or the phase constant, the skin depth and thus the conductivity or electrical conductivity of the neighboring formations to be determined.

By suitable choice of the constants of the sub-

i ^r> suchungssystems, ie the antenna Spacings- or spacings and the frequency can desired radial and vertical solutions can be realized, which permit the examination of each interest formation zone. The above examples also show how to select the appropriate spacings or distances and the frequency for the investigation of the invasion zone and the invasion-free zone. According to the teaching of the present invention, of course, any other zone or combination

> ·> Nation of zones of interest through suitable Selection of spacings and frequency in accordance with the teachings of the present invention to be examined.

For this purpose 4 sheets of drawings

Claims (2)

Patent claims: 1. Geophysical logging equipment for examining earth formations that have been drilled through a borehole with a probe movable through the borehole, the one transmitting antenna and at least two Receiving antennas at a distance from one another with downstream measuring and evaluation devices having, and with a generator device for feeding the transmitting antenna with high-frequency electromagnetic energy, which is radiated into the surrounding earth formations, a frequency being selected at which the radiation energy propagates at least in the invasion zone, characterized by the following features: a) the distance (L _(r ) between the transmitting antenna (T) and the center of the distance (. 1 /) between the receiving antennas (R 4, R5) is at least ^ times the borehole diameter and is at least half as large as the maximum » Skin depth «of the invasion zone; b) the distance (AL) between the receiving antennas (R 4, RS) is less than 3 times the minimum "skin depth"; c) the measuring and evaluation devices (25 to 26, 51 to 54) are those for determining of phase and / or amplitude gradients are known per se. 2. Device according to claim 1, characterized by circuits for exciting the transmitting antenna (T) with variable frequency, by circuits connected to the receiving antenna (R) for measuring at least one factor of the propagation constant of at least part of the earth formation (21) surrounding the borehole (19) ) for generating a corresponding output signal (38 a), by circuits (38) responding to this output signal for generating a control signal for readjusting the variable frequency in such a way that the measured propagation constant factor is kept at a desired value, and by circuits designed to respond to the control signal (29) for generating a corresponding measurement value for the conductivity of the earth formation surrounding the borehole.