Magnetics

Most of the background information presented here on the magnetic method was compiled from M. P. Dobrin's Introduction to Geophysical Prospecting, and readers who desire an even more detailed discussion should refer directly to that text.

The Earth's Magnetic Field
The earth possesses a magnetic field caused primarily by sources in the earth's core. The field is a vector field with magnitude and direction similar to a field caused by a dipole or bar magnet located near the earth's center and aligned subparallel to the geographic axis. The intensity of the earth's magnetic field is expressed in S.I. units of nanoTesla (nT) or in an older equivilent unit, the gamma.

The intensity of the earth's magnetic field varies between 45,000 and 60,000 nT over the coterminous United States. If geophysical measurements were made with a magnetic needle it would be found that the north-seeking end of the needle dips downward in the northern magnetic hemisphere, while in the southern magnetic hemisphere, the south-seeking end will dip downward. In between, at the magnetic equator, the needle will be horizontal; i.e, the inclination is zero. As the needle is moved closer to the magnetic poles, the angle of inclination increases from zero degrees at the equator to ninety degrees at the poles.

Temporal Variations in the Earth's Magnetic Field
The earth's magnetic field varies with time. Temporal changes can be resolved into secular changes, solar diurnal changes, lunar diurnal changes, and changes resulting from magnetic storms. Secular changes take place over decades and centuries and do not impact short term magnetic measurements in geophysics.

The remaining higher frequency variations are significant. Normal diurnal variations have a period of about one day and an average amplitude of 25 nT; however, changes as large as 1000 nT may be observed during magnetic storms. Magnetic surveys are usually halted during magnetic storms. Diurnal variations are unpredictable but normal variations can be measured and removed from magnetic observations.

Buried Magnetic Bodies
Variations in the magnetic field of the earth which are attributable to changes in geologic structure, magnetic properties in near-surface rocks, or man-made structures are measured with the magnetic method. Many rocks and minerals are weakly magnetic or magnetized by induction in the earth's magnetic field and cause spatial variations in the earth's field. The induced magnetization is in the direction of the external field, and its strength is proportional to the strength of the external field.

The constant of proportionality between the external field and the induced field is called the susceptibility. The susceptibility is zero for a vacuum and entirely nonmagnetic substances. Magnetic materials having positive susceptibilities are known as paramagnetic materials. A paramagnetic material of very high susceptibility is said to be ferromagnetic. Man-made objects containing iron or steel are often ferromagnetic and can create changes in the earth's field of several thousand nT.

Magnetic Measurements
A widely used land magnetometer is the nuclear resonance (proton) magnetometer. Most chemical elements have a magnetic moment; each nuclei represents a small spherical magnet spinning about its magnetic axis. Spheres of this type orient either parallel or antiparallel to an external magnetic field with the majority of the nuclei oriented in the antiparallel direction. When an external magnetic field is applied then removed the magnetic moment returns to its original orientation (that of the earth's field) by precessing around the ambient field with an angular frequency that is proportional to the magnitude of the earth's magnetic field. Proton magnetometers measure the precession frequency and calculate a value for the magnitude of the earth's magnetic field. Proton magnetometers measure the total magnetic field of the earth rather than the individual components.

Similiarly, optically pumped magnetometers have come into widespread use. They exhibit faster cycling, improved noise resistance, and increased precision.

The measurement of magnetic gradient is a useful technique in magnetic exploration. Magnetic gradients are not affected by diurnal variations and serve to emphasize the anomalies caused by shallow bodies. Land survey gradiometers employ two individual magnetometers separated in the direction of the desired gradient measurement by a distance of approximately one-half meter. The gradient is approximated by the difference between the values measured by the two sensors, divided by the sensor separation.

The magnetometer is operated by a single person. However, grid layout or safety concerns may require the presence of another technician. The incorporation of computers and non-volatile memory in magnetometers has greatly increased the ease of use and data handling capability of magnetometers. The instruments can typically record position, comments, and an entire day's worth of magnetic data. The operator can have a significant effect if watches, knives, keys and other ferrous objects are not removed from his or her person.

Interpretation of Magnetic Data
Much of the information obtained from magnetic data is interpreted in a qualitative sense. The interpreter of shallow-target magnetic data is often concerned with the presence or absence of a subsurface feature rather than its depth or shape. However, following anomaly identification, a more detailed interpretation effort can yield the following quantitative pieces of information; the magnetic susceptibility contrast of the causative body, the dimensions of the body, and the depth of the body. While no interpretation is unambiguous, a reasonable interpretation is found by limiting the geophysical solutions with known geologic and physical constraints.

An iterative process of modeling magnetic data aids the geophysical interpretation. Synthetic magnetic data is compiled (based on an input model) and compared with the observed magnetic data. Errors in the initial model are refined and the process begins again. Hopefully, within several interations the error between the model data and observed data will be small enough to confidently conclude that the causative body is similar in size, depth, and susceptibility to that of the model body.

The declination and inclination of the earth's magnetic field greatly influence the shapes of magnetic anomalies. Total magnetic field anomalies are highly variable in shape and amplitude; they are almost always asymmetrical, appear complex even when related to simple sources, and usually are the combined effect of several sources. The observed magnetic anomaly is the sum of the earth's magnetic field and the magnetic field induced into the source body. The geophysical anomaly is positive when the field of the buried body reinforces the earth's field and is negative when the field opposes the earth's field. The inclination of the earth's field is approximately 60 degrees, so most magnetic bodies will create a dipolar total field anomaly. A magnetic low is expected near the north end of the body and a magnetic high is expected near the south end of the body. An anomaly from a shallow object will be sharper laterally (higher spatial frequency) and larger in amplitude than an anomaly from the same object buried at a greater depth.

Steel underground storage tanks (USTs) can create magnetic field anomalies as large as a few thousand nanoTesla (nT) while a concrete reinforced septic tank may create a geophysical anomaly of only 100 nT. The spatial character of a magnetic anomaly is as important as anomaly magnitude. A long linear total field feature is probably attributable to a pipe, foundation, or roadway rather than an underground tank. It is also necessary to integrate any possible external information into the interpretation, whether it is in the form of historical information or an interpretation from a different geophysical method. It is important to separate anomalies caused by cultural features such as debris piles, pipes, and buildings from subsurface related anomalies. Field maps of cultural features enable the identification of cultural anomalies.

Micro Geophysics Corporation (MGC) has recorded hundreds of thousands of magnetic readings for environmental and mining applications. We have a variety of sotware which can be used to model the geohysical fields produced by your suspected subsurface configuration. We offer the caculation of the anomalous total field due to one simple geometric body if you will contact us with the geometry and location of the body. Just fax (303-424-0807) or E-mail (microgeo@aol.com) or phone (1-800-GEOPHYSics) to us the lattitude, orientaion of the body with respect to magnetic north, its size, shape and composition and we will respond with the contoured effect of the body. The results may surprise you!!