Procedure and device for the detection of inversions of the earth's magnetic field by means of measurement taken in a drill shaft

A method and device for detecting inversions of the earth's magnetic field by taking measurements in a drill shaft. The device includes a first probe used to measure the magnetic induction B.sub.1 during its movement in the shaft, a sensor to measure magnetic induction B.sub.2 on the surface of the ground at a stationary point at the same time as B.sub.1 is measured, a second probe to measure magnetic susceptibility X of the rocks as it moves through the shaft, and a data-recording and data-processing unit. The data-recording and data-processing unit further includes a device for calculating a value of B.sub.1 -B.sub.2, filtering the calculated value to isolate the induction fraction B.sub.L, which is only composed of a component B.sub.I resulting from an induced magnetization, and the component B.sub.R resulting from the residual magnetization. The induction B.sub.I based on the induction B.sub.1 in the shaft and on the magnetic susceptibility X can be calculated.

The present invention concerns a procedure making it possible to determine 
the residual magnetization of rocks through which a shaft is drilled, in 
order to locate the zones of inversion of the earth's magnetic field. 
The earth's magnetic field is now oriented northward; however, over the 
course of geological time, it very often swung between the north and the 
south. Rocks possess a magnetization which may be separated into two 
terms: one portion, induced by the earth's magnetic field, is called 
induced magnetization; the other part corresponds to the storage of the 
earth's magnetic field as it existed when the rocks were deposited and/or 
formed, and it lies in the direction of this terrestrial magnetic field. 
This latter fraction is called residual magnetization. It may be supposed 
that the inversions of the direction of residual magnetization found 
during drilling in successive rock layers faithfully mirrors the 
inversions of the terrestrial magnetic field produced between the epochs 
during which the layers were deposited. 
The detection of inversions thus makes it possible to date the rocks. In 
the field of oil well drilling, knowledge of the age of the rocks is of 
the highest importance, since it allows a correlation to be established 
among the layers. 
Procedures for the detection of inversions of the earth's magnetic field 
are already known. The most widely-used procedure consists in taking 
multiple samples of rocks along the drill shaft, bringing them up to the 
surface, and conducting a magnetic analysis in the laboratory in order to 
determine their residual magnetization. However, this procedure is long 
and expensive; once brought to the surface, moreover, the samples acquire 
an interference magnetization due to the influence of terrestrial 
magnetization and of temperature change. 
For this reason, an entire family of magnetic measurement probes was 
created in the past, called magnetometers or magnetoradiometers, which 
make it possible to determine at the actual site the magnetization of the 
rocks as the probe moves through the drill shaft, this probe being 
connected to a data-processing unit installed on the surface. 
Examples of such probes are described in Patents GB-A-2 158 950, U.S. Pat. 
No. 3,453,531, and U.S. Pat. No. 3,965,412. When working on highly 
magnetized rocks, the induction created in the well by residual 
magnetization is sufficiently pronounced to be detected directly. However, 
with respect to oil drilling, the rocks exposed to the probe have a very 
low degree of magnetization, with only rare exceptions. Now, the value of 
the magnetic induction must be known with a high degree of precision. 
Since the inductions involved range approximately from 1 to 10 nanoTeslas 
(nT), it is desirable that they be known with an accuracy of less than 1 
nT. 
Very precise tri-axial magnetometers are known which make it possible to 
determine magnetic induction by reconstruction based on the three 
components. However, the lack of precision permitted by this technique 
makes the operation difficult in the area of oil drilling. This lack of 
precision is not linked to the intrinsic features of the magnetometer, but 
rather to the detection procedure. 
U.S. Pat. No. 4,071,815 disclosed a procedure for the detection of 
inversions of the magnetic field, which consists in conducting two 
simultaneous measurements of the total magnetic field B.sub.1 and B.sub.2 
in the drill shaft at two separate points, and two measurements of the 
terrestrial magnetic susceptibility X.sub.1 and X.sub.2 at the same 
points; in calculating the difference B=B.sub.1 -B.sub.2 and the 
difference X=X.sub.1 -X.sub.2 ; and finally, in calculating the value 
R=Difference B-Difference X, which is representative of the fraction of 
residual magnetization present in the rock at these two points. 
This known procedure is useful, but it suffers from a lack of optimization 
which would make it possible to exploit the value R which, as indicated 
above, contains only a fraction of the information pertaining to residual 
magnetization. 
The purpose of the present invention is to overcome the disadvantages of 
the procedures belonging to prior art, described above. The invention 
pertains to a procedure for the reading of measurements of the magnetic 
field in a drilling well, and for the processing of these measurements so 
as to determine, in concrete fashion, the zones of inverted and normal 
polarity through which the drill shaft passes. A zone of normal polarity 
is one in which the residual magnetization in the rock is oriented in the 
same direction as the present-day terrestrial magnetic field; inversely, 
in a zone of inverted polarity, the residual magnetization of the rock is 
out of phase by 180.degree. in relation to the present-day terrestrial 
magnetic field. 
The procedure according to the invention is characterized by the fact that 
it consists in: 
measuring the variations in the terrestrial magnetic induction B.sub.1 at a 
large number of points along the entire length of the drill shaft by 
continuously bringing to the surface a measuring apparatus and, 
simultaneously, measuring the variations in the magnetic induction B.sub.2 
at a fixed point on the surface of the ground, as well as the variations 
in the magnetic susceptibility X of the rocks at a large number of sites 
along the entire length of the drill shaft, by bringing the measurement 
apparatus continuously to the surface; 
determining the value of B.sub.1 -B.sub.2, in order to eliminate the 
influences of the temporal variations in the terrestrial magnetic field on 
the magnetic induction B.sub.1 measured in the shaft; 
filtering said quantity, in order to eliminate the induction fraction 
resulting from distant magnetic sources, and thus to isolate the remaining 
induction fraction B.sub.L, which comprises a component B.sub.I resulting 
from induced magnetization and a component B.sub.R resulting from residual 
magnetization; 
calculating the induction B.sub.I based on the measurements of the 
induction B.sub.1 and of the magnetic susceptibility X; 
and finally, calculating, based on the inductions B.sub.L and B.sub.I thus 
obtained, the influence of the changes of orientation of the residual 
magnetization. 
The invention also pertains to a device making it possible to effect the 
processing of the data obtained concerning the inductions B.sub.L and 
B.sub.I, in order to calculate the direction of the residual magnetization 
.

As explained above, measurement is made, first, of the terrestrial magnetic 
induction B.sub.1 and of magnetic susceptibility X along the drill shaft, 
and second, the magnetic induction B.sub.2 at surface level. 
It is known that the induction B.sub.1 takes the form: 
EQU B.sub.1 =B.sub.O +B.sub.L +B.sub.T' 
where 
B.sub.O is the induction linked to distant magnetic influences; 
B.sub.L is the induction linked to proximate magnetic influences (induced 
and residual magnetization); and 
B.sub.T is the induction linked to the temporal variation of the 
terrestrial magnetic field. 
It is obvious that induction B.sub.2 at the surface takes the form B.sub.2 
=B.sub.T, given that the influence of distant and proximate sources is 
negligible. 
The first processing operation at the surface consists of isolating the 
parameter B.sub.L. For this, the following is computed: 
EQU B.sub.1 -B.sub.2 =B.sub.O +B.sub.L +(B.sub.T -B.sub.T'). 
For rocks possessing low conductivity (which is the case in the field of 
oil geology), B.sub.T and B.sub.T' can be identified without deterioration 
of results up to a depth of 2,000 meters. Beyond that point, the 
attenuation produced by the layers lying above it must be considered. 
Because of its origins, induction B.sub.O possesses variations of a longer 
wavelength than induction B.sub.L. By filtering appropriately, B.sub.L and 
B.sub.O can thus be suitably separated. Thus, B.sub.L is made available 
with a high degree of precision. It will be remembered that the 
information contained in the quantity B.sub.L pertains to the induced 
magnetization B.sub.I and partly to residual magnetization B.sub.R. 
Moreover, using the measurement of B.sub.1, the induction B.sub.I created 
by the induced magnetization of the rocks through which the shaft is 
drilled can be determined with great accuracy, using the formula: 
EQU B.sub.I =XB.sub.1 k, 
where k is the connection factor between X and the induction in the well, 
which depends on the geometry of the formation and of the well. 
Beginning with this stage, two techniques can be applied to detect the 
residual magnetization inversions; these techniques rely on theoretical 
and experimental arguments. 
One of these techniques consists in tracing the logging surveys of B.sub.L 
and B.sub.I (in nT) as a function of the depth z (in meters). FIG. 1 
represents an example of these logging surveys. As can be seen, the peaks 
and valleys of B.sub.L and B.sub.I have a given periodicity (of 
approximately one meter, for example). A peak is found to correspond to an 
increase in the magnetization in the formation. 
For each of the peaks, point A' where the induction is lowest is taken as 
the point of origin of the curve B.sub.L and the corresponding point A at 
the same depth, as the point of origin of curve B.sub.I. The cycle of 
curve B.sub.I is subjected to a displacement AA', thereby causing point A' 
to coincide with A. Several configurations may then appear, only some 
examples of which will be considered below with the interpretations 
resulting for a configuration of the magnetic fields identical to that 
found in the Parisian Basin. It should be noted that, for other magnetic 
field configurations encountered in other parts of the globe, 
interpretations will differ but will be based on the same curve-analysis 
principle. 
1. The curve B.sub.I lies above curve B.sub.L (FIG. 2a): the increase in 
B.sub.I is attenuated by B.sub.R. The residual part B.sub.R contained in 
B.sub.L is thus opposed to B.sub.I ; the residual magnetization in the 
formation is opposed to the induced magnetization. The residual 
magnetization is said to be inverted. 
2. Displaced curve B.sub.I merges with curve B.sub.L (FIG. 2b): the 
direction of B.sub.R is indeterminate. 
3. Displaced curve B.sub.I lies beneath curve B.sub.L (FIG. 2c), signifying 
that the increase in B.sub.I is strengthened by B.sub.R. As a result, the 
residual magnetization occurs in the direction of the induced 
magnetization. The residual magnetization is termed normal. 
The same procedure is followed for each peak of the logging surveys B.sub.L 
and B.sub.I. All of the results obtained can thus be summarized in the 
diagram in FIG. 3, in which the depth z lies along the abscissae and the 
direction of B.sub.R, along the ordinates. 
The second technique for detection of inversions of residual induction 
begins with the induction curves B.sub.L and B.sub.I and consists in 
calculating the absolute values of the gradients or of the first 
derivatives: 
##EQU1## 
for an appropriate base dz. This calculation makes it possible to avoid 
the large general variations of these inductions resulting from the 
lithology of the rocks, and pinpoint only localized variations. In this 
way, two curve profiles GB.sub.L and GB.sub.i, as illustrated in FIGS. 4a 
and 4b, are obtained. 
For each of the curves GB.sub.L and GB.sub.I, families of curves L.sub.i 
and I.sub.i are calculated and put in parametric form as a function of the 
second derivative of B.sub.L and of B.sub.I. 
As an example, the upper-limit curve L.sub.n will pass through the maximum 
values of GB.sub.L ; curve I.sub.n will pass through the maximum values of 
GB.sub.I as shown in FIG. 4a. The curves L.sub.i and I.sub.i are then 
recorded, and the following ratio is calculated, where K.sub.i corresponds 
to the curves L.sub.i and I.sub.i, i.e., the curves passing by the points 
where dGB.sub.L /dz=0 and dGB.sub.I /dz=0, respectively: 
##EQU2## 
As another example, the ratio of the absolute values of the maximum curves 
is calculated, where K.sub.n corresponds to the curves L.sub.n and 
I.sub.n, i.e., the curves passing by the points where dGB.sub.L /dz=0, and 
dGB.sub.I /dz=0, respectively: 
##EQU3## 
The value of K.sub.i will be greater or less than 1, according to the 
relative position of the curves L.sub.i and I.sub.i. 
In a magnetic configuration identical to that encountered in the Parisian 
Basin, if K.sub.i &gt;1, the residual magnetization is normal (i.e., in the 
direction of the induced magnetization). In other magnetic configurations 
encountered on the surface of the globe, the conclusions could be 
different. 
It will be noted that the display of K.sub.i is better when its logarithm 
is represented. The polarity of B.sub.R is determined as a function of the 
sign of the log (K.sub.i). 
The above-mentioned measurements and calculations are performed by a 
processing device illustrated schematically in FIGS. 5a and 5b. This 
device comprises a first probe 10 incorporating a sensor 12, for example, 
a nuclear magnetic resonance sensor, enclosed in a packing 14 used to 
measure magnetic induction B.sub.1 in the drill shaft 16 as it moves in 
said shaft; a sensor 18, for example a nuclear magnetic resonance sensor, 
used to measure the magnetic induction B.sub.2 on the surface of the 
ground 21 at a stationary point at the same time as the measurement of 
B.sub.1 is being made; a second probe 20 comprising a transmitter 22 and a 
sensor 24 enclosed in a protective envelope 26 and used to measure the 
magnetic susceptibility of the rocks in the drill shaft during its 
movement in that shaft; and a data-recording and data-processing unit 28 
comprising means for filtering the value B.sub.1 -B.sub.2 for the purpose 
of isolating the induction fraction B.sub.L containing only the component 
B.sub.I resulting from induced magnetization and the component B.sub.R 
resulting from residual magnetization; and means for calculating the 
induction B.sub.I based on the induction B.sub.1 in the shaft and on the 
magnetic susceptibility X. In FIG. 5b, dotted lines represent the lines of 
the magnetic field 30. 
To apply the first detection technique described above, the data-recording 
and data processing unit 28 also includes a device comprising means for 
tracing curves B.sub.L and B.sub.I, means for subjecting each peak of 
curve B.sub.I to a displacement AA' which causes point A of curve B.sub.I 
having a lower value to coincide with the corresponding point A' of the 
curve B.sub.L, and means for comparing the relative changes in the curves 
B.sub.L and B.sub.I positioned in this manner. 
To implement the second technique, according to the invention, the 
data-recording and data processing unit 28 further includes a means for 
calculating the first derivatives of the inductions B.sub.L and B.sub.I ; 
means for tracing the absolute values of the derivated curves GB.sub.L and 
GB.sub.I as a function of depth (the depth base being capable of 
variation); means for calculating and tracing the families of curves 
L.sub.i and I.sub.i and placing the curves L.sub.i and I.sub.i forms as a 
function of the second derivatives of curves B.sub.L and B.sub.I ; (at 
various depths) and means for calculating the ratio: 
##EQU4## 
and for studying its value as a function of depth.