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Downhole magnetic surveys
Downhole magnetic surveys have been most commonly applied in highly magnetized igneous rocks, which have usually been studied within pure geoscience, especially beneath the ocean floor. These rocks preserve the direction of the Earth’s field at the time of their formation (i.e., the prevailing magnetic field is "frozen" in the rocks as they solidify, giving them a strong natural remnant magnetization). A primary application has been to identify points in time at which the Earth’s magnetic field has undergone a polarity reversal. These reversals have been dated globally (e.g., isotopically in the case of volcanic series or by correlation with biostratigraphy in the case of volcaniclastics) and have given rise to a geomagnetic polarity time scale (GPTS) that is based on laboratory measurements. This, in turn, has allowed dates to be assigned to a given magnetozone that is bounded by reversal phenomena. It has been possible to recognize these reversals through downhole measurements and, therefore, to date the rocks accordingly.
Sedimentary rocks have much weaker remnant magnetizations than igneous sequences, and it has been much more difficult to investigate their magnetic character. However, recent advances in instrumentation have led to progress in downhole magnetic measurements of sedimentary strata.
The following magnetic theory is extracted from Lalanne et al. The magnetic field measured downhole has three parts: the Earth’s magnetic field of the present day; the field that is induced in the rocks by the prevailing Earth’s field; and the remnant magnetic field, which is the preservation in the rocks of a paleomagnetic field. The effect of the Earth’s magnetic field can be accommodated during logging by extrapolating downhole the measurements made by a surface magnetometer that records diurnal variations in the Earth’s field and allows the downhole data to be corrected for these variations where they are significant. The induced field is proportional to the magnetic susceptibility of the rock, which is governed by (ferro-magnetic) mineralogy and fluid composition. The remnant magnetic field adopts the direction of the Earth’s field at the time that the rock was forming. For sediments, it is most pronounced in clays.
A measurement of magnetic induction or field strength, BT, can be written as
where Bo is the magnetic induction associated with the present Earth’s field, Bi is the magnetic induction caused by the field induced in the rock, and Br is the magnetic induction caused by the remnant field. Magnetic induction is measured in units of nanoTesla (nT). It is a measure of field strength expressed in terms of the field’s ability to induce magnetization. Typically, Bi and Br are no more than a few tens of nanoTesla, and they have to be measured against a prevailing Earth’s field that is a thousand times greater. Therefore, the exercise becomes very much one of analyzing residuals. For this reason, the prevailing Earth’s field, Bo, is removed from the value of BT, which then becomes a "net" field Bt.
The induced magnetic field, Bi (nT), is proportional to the magnetic susceptibility, χ (units SI), of the rock:
where μo is the magnetic permeability of the void (4π × 10–1 μH/m), and HT is the Earth’s magnetic field (A/mm). Therefore, Bi can be evaluated if susceptibility can be measured.
If Bt and Bi can be determined, Br can be quantified,
If Br is positive, the remnant magnetization and, therefore, the paleomagnetic field that caused it are aligned as per the present-day Earth’s field, and Br is described as "normal." Otherwise, its polarity is "reversed." The polarity of remnant magnetization is evaluated through a numerical comparison of Bt and Bi.
A magnetic logging sonde developed for sedimentary rocks is Schlumberger’s Geological High-Resolution Magnetic Tool (GHMT™). It actually comprises two tools: one to measure the total magnetic field and one to measure magnetic susceptibility. The tool housings are nonmagnetic and electrically insulating with a diameter of 4 in. [100 mm].
Because sedimentary rocks have a very low magnetization, a very high precision magnetometer is required. This requirement is satisfied by Schlumberger’s Nuclear Magnetic Resonance Tool (NMRT™), which uses the principles of nuclear magnetic resonance whereby the frequency of precession of relaxing protons is proportional to BT (see NMR applications). The problem, therefore, reduces to a very precise measurement of frequency. The NMRT measurement has a sensitivity of 10–2 nT. Data are recorded at a logging speed of 1970 ft/hr [600 m/hr] with a sampling interval of 4 in. [100 mm].
A second tool measures the magnetic susceptibility of the rock, which is proportional to Bi (Eq. 3). This tool, Schlumberger’s Susceptibility Measurement Tool (SUMT™), uses the principles of electromagnetic induction. The voltage induced in the receiver coil increases with susceptibility, which is determined from the complex character of the induced signal. Susceptibility is dimensionless. The downhole measurement of susceptibility has a sensitivity of approximately 10 –6 units SI. Data measured in sedimentary rocks are typically in the range 10 –5 to 10 –4 units SI. Data can be recorded at a logging speed of up to 3940 ft/hr [1200 m/hr] with a sampling interval of 6 in. [150 mm]. The tools are rated to a temperature of 257°F [125°C] and a pressure of 15,000 psi [103 MPa].
The key measurement deliverable from the combined use of these two integral tools is a depth record of the polarity of remnant magnetization based on the sign of Br in Eq. 4. This display is called the well magnetic stratigraphy (WMS).
Fig. 3 illustrates the application of the measured data. It shows how a magnetostratigraphic sequence was established for a well in the Paris basin using GHMT log data. For dating purposes, this sequence has to be tied to the GPTS.
Fig. 3 – Comparison between the Geomagnetic Polarity Time Scale (GPTS) and the Well Magnetic Stratigraphy (WMS) for a borehole in the Paris basin. Normal polarity is black; reversed polarity is white. The GPTS and WMS are similar. This allows polarity to be correlated and absolute determinations of age to be made along the borehole column. In some places (e.g., at approximately 950 m depth), the detail of the GPTS is not reflected in the WMS, presumably because of low rock magnetization or, possibly, localized erosion. On the other hand, at approximately 1060 m, there are indications of detail that has yet to be accommodated within the GPTS. (Courtesy of SPWLA).
Fig. 4 illustrates another example, this time from the Ocean Drilling Program (ODP). This example reveals some geomagnetic features that are not yet part of the global standard. Notwithstanding these disparities, this chronal benchmarking allows the absolute dating of much of the sedimentary succession.
Fig. 4 – Geomagnetic data from Ocean Drilling Program (ODP) Leg 145. Wells 884B and 884E are 24 m apart. Normal polarity is black; reversed polarity is white. Well 884B was cored; cored data have furnished the inclination of the remnant magnetization. These data have been correlated with the Geomagnetic Polarity Time Scale (GPTS). Well 884E was logged geomagnetically; log data have furnished the Well Magnetic Stratigraphy (WMS). The WMS shows detail that is not part of the GPTS. This could be highly significant because in these wells the susceptibility was very low indeed, so that the measured “net” field Bt was directly related to the remnant magnetization without being impacted by possible errors in susceptibility. (Courtesy of SPWLA.)
Future applications will examine the direction of remnant magnetization to investigate the movement of fault blocks and enhancing the fieldwide correlation of the sedimentary column.
|Bo||=||magnetic induction associated with the present Earth’s field, m/qt, nT|
|Bi||=||magnetic induction due to the field induced in the rock, m/qt, nT|
|Br||=||magnetic induction due to the remnant field, m/qt, nT|
|Bt||=||"net" field, m/qt, nT|
|HT||=||Earth’s magnetic field, q/tL, A/mm|
|T||=||tension, mL/t2, N|
|χ||=||magnetic susceptibility of the rock|
- Pages, G., Barthies, V., Boutemy, Y. et al. 1994. Wireline Magnetostratigraphy Principles and Field Results. Presented at the SPWLA 35th Annual Logging Symposium, 1994. SPWLA-1994-XX.
- Lalanne, B., Bouisset, P., Pages, G. et al. 1991. Magnetic Logging: Borehole Magnetostratigraphy and Absolute Datation in Sedimentary Rocks. Presented at the Middle East Oil Show, Bahrain, 16-19 November 1991. SPE-21437-MS. http://dx.doi.org/10.2118/21437-MS.
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