(Courtesy Alaska DNR DGGS gpr_2001_01a_sh001)
EARTHS MAGNETIC FIELD:
The magnetic field found at any point on or within the earth is comprised of the sum of the superimposed fields caused by induced and remanant magnetism.
The image, above is a portion of a contour map representing Residual Magnetic Intensity, RMI.
When a material having magnetic susceptibility is placed in a magnetic field, induced magnetism occurs. The strength of the induced magnetic field is proportional to the susceptibility and the applied magnetic field. The total magnetic field increases because the induced field is of the same polarity or sign as the applied field.
Remanant magnetism can be seen when the applied magnetic field is removed. An interesting example is volcanic rock that is formed by molten lava. As the molten lava cools, it retains the induced magnetic field that was present at the time of cooling. When volcanic rock is measured for its magnetic properties of polarization, it may be in the same direction as the magnetic field found on the earth today or it may be reversed in direction.
MAGNITUDE OF THE FIELD:
The magnitude (intensity) of the magnetic field of the earth is about 60,000 nano Teslas (nT) or 60,000 gamma (g). The surface of the earth has a very complex variation of magnetic field which is caused by crustal rocks and variation in iron content having remanant and induced magnetization. Geologic structure may be inferred from measurements of the total magnetic field using an instrument called a magnetometer. Magnetometers are used in both ground and airborne applications. Other applications are found using magnetometers in a borehole environment.
Magnetic properties of rocks and other materials may be measured directly within a borehole. Magnetic susceptibility is directly related to the iron content as a percent of total volume in rocks and minerals. This measurement is of great value when correlation can be made to iron ore grade or the abundance of magnetite as an indicator of redox (reduction-oxidation) reactions within mineralized environments.
It was noted above that the total magnetic field is comprised of the sum of the remanant magnetic field, (Mn) and the induced magnetic field, (Mi). The property of magnetic permeability, (m) of most rocks is close to 1.0. Rock magnetic susceptibility which is considered to be the induced component has a magnitude that is expressed in terms of x where: x = 1 - m (cgs).
The Koenigsberger ratio is defined as Q where:
Q = Mn/Mi = Mn/xH
Data has been cited in Strangway (1967) that give value of Q for various rock types. For example, Values of Q can range from 0 to 1 in Mafic flows, exceed 1.0 in sills and dikes and be as high as 100 in basalt flows.
Reference: Strangway, D. W.; “Magnetic Characteristics of Rocks,” in Mining Geophysics, Vol. II, Soc. Expl. Geophys., Tulsa, Okla. , 1967.
Various minerals and metals located within the earth offer differing responses to induced magnetic fields. The subject of magnetic susceptibility is covered on another page on this website. Learn more about magnetic susceptibility.
Flux density (B) is proportional to the magnetizing force (H). The relationship of Flux density to magnetic force and magnetic permeability may be expressed as follows:
B = mH
Where B is flux density measured in units of Webers (Wb) per meter2.
As referenced above, magnetic permeability, (m) of rocks is close to 1.0. The permeability of vacuum is denoted as m0.
In the units Systeme Internationale (SI), based on the meter, kilogram, second, and ampere, m0 = 4p * 10-7 Ohm-Sec/m.
Reference: D.S. Parasnis, Mining Geophysics, Second edition, Elsevier Scientific Publishing Company, 1973
TYPES OF MAGNETOMETERS:
Magnetometers are available in the following common types.
Flux gate magnetometers
THE GEOMAGNETIC FIELD:
Prospecting using the magnetic method requires measurement of areas in which the earth’s magnetic field is distorted. Therefore, it is of importance to know what the normal magnetic field is for a given area for comparison. When viewing the totality of the earths magnetic field at any given point on the surface, it is also important to recognize that the total field is a sum total of all combined magnetic fields or magnetic vectors. The total field is comprised of the horizontal (H), vertical (Z), and declination of the horizontal field (D of H) east or west of true north.
Maps are available that depict total magnetic field over the surface of the earth.
The magnetic field of the earth is under constant change. Although the change is small, it can affect the results obtained in field measurements. It is therefore common practice to establish a reference field or base station magnetometer to measure the local diurnal change in the magnetic field at least during the period that a magnetic survey is being conducted. When observed over a 24 hour period, diurnal variation can exceed 100 g.
Given A Normal vertical field intensity of 50,100 g, if the base station located in a magnetically neutral zone reads 50,150 g, then it is apparent that the diurnal effect on local observations is contributing 50 additional g of magnetic intensity at the moment in time that the survey was conducted. A correction factor – subtracting 50,150 g is applied to the field survey measurement to establish a “working zero”.
WELLOG HAS MAGNETOMETERS:
OEM 3 AXIS MAGNETOMETER:
A three axis magnetometer can be adapted to produce heading plus two axis tilt. Applications include borehole surveys for inclination and azimuth determination. This type of borehole survey is often referred to as a deviation survey. Magnetometer surveys can be conducted for the purpose of measuring the proximity of geophysical anomalies. Anomalies can be caused by nearby mineralized ore bodies.
The three axis magnetometer can be piggybacked on a microcontroller board for data logging in boreholes and on the surface.
WELLOG has sensitive, small, low cost solid-state magnetometers to measure borehole magnetic fields and surface magnetic fields in 3 axes.
This magnetometer can be used with a laptop or with an optional LCD display module
Revised 11-07-2016 © 2007-2016 WELLOG All Rights Reserved