Understand Magnetic Maps 1 May 2006 Magnetic maps contain valuable information that is unknown unless one has some understanding of the fundamental processes that create the patterns in those maps. This report is intended to aid that understanding. A casual and uninformed look at magnetic maps will indeed tell much about the shape and location of buried features. However, an informed study of a map can provide additional details about the depth, quantity, and identity of the magnetic materials that are underground. Basic knowledge of magnetic maps may also prevent foolish interpretations and wasteful excavations. The ideas in this report fall between those of a magnetic survey and its analysis; these bordering topics receive some discussion here, but that part of the report is not very thorough. Some considerations for the processing and interpretation of magnetic data are included; also, some of the ideas here may help with decisions about field work. This report is primarily for individuals who do magnetic surveys for archaeological applications; however, some of the topics may aid others who have different goals. There are only a few illustrations of magnetic measurements in this report (most illustrations have been calculated); this is because calculated maps isolate the important factors with greater clarity. An excellent compilation of magnetic maps of archaeological features has recently been published by Tatyana Smekalova (Smekalova, Voss, and Smekalov 2005); that publication also includes an archaeological analysis of those magnetic maps, a topic that is lacking in my report. The best overall introduction to magnetic surveys and their understanding remains the publication by Sheldon Breiner (Breiner 1973); my report is designed to supplement some parts of Breiner's publication. The appearance of magnetic maps changes with location on the Earth; latitude has the greatest effect. The maps that illustrate this report are typical of ones that can be measured in most of the USA, northern Europe, Australia, and South Africa; for most of the calculated maps here, the angle of inclination of the magnetic field is assumed to be 70°. In other parts of the world, there can be significant differences in the appearance of magnetic maps. These differences are mentioned here, but Breiner (1973) has a more complete description of these latitude effects. While this report mentions gradiometers, most of the illustrations and discussions are for total field magnetic surveys. Topics generally get more detailed later in the report, and also later within each section. When one paragraph has more details than usual, the word "(technical)" is put at the start of the paragraph. The electronic version of this report has hyperlinks, primarily to figures; these are indicated with blue text. The captions for the figures are detailed, and the most important information is with those figures (which are at the end of the report). If the report is read by looking at the figures, the initial blue text (Figure ##) in a caption has a link back to the primary discussion in the body of the report. Additional information about the figures is included in an appendix. Where a full page of figures has four panels, an individual panel Page 1 Preliminaries may be enlarged by clicking on it. The main sections of this report are as follows: Preliminaries Different Styles of Magnetic Maps General Effects in Magnetic Maps The Magnetic Lows Induced and Remanent Magnetization Data Processing Analysis of Magnetic Maps The Components of the Magnetic Field Conclusion Preliminaries A magnetic map illustrates changes in the magnetic field in an area. Objects that are underground can warp the simple patterns of the Earth's magnetic field into complex shapes. A study of these shapes on a magnetic map can reveal much information about the features that are underground. This information can include the location, size and shape, volume or mass, and depth of the features; in some cases, the age of a feature and its material (stone, soil, metal) may be estimated. Magnetic maps are created from numbers, often measured at a uniform interval in an area. Figure 1 shows a map that has a group of numbers in their correct spatial locations. Magnetic measurements are made with a magnetometer. There are many different types of magnetometers, and they are often given a prefix that describes a fundamental physical aspect of their operation: Overhauser, cesium, fluxgate, proton. All of these types of magnetometers are excellent for archaeological surveys. Each of these magnetometers measures the amplitude (also called the magnitude) of the Earth's magnetic field; this is complementary to a magnetic compass, which measures direction, but not amplitude. The technical name for this amplitude is flux density; in physics and engineering books, this name is designated with the letter B. The typical unit for this quantity is the nanotesla. The "nano" means billionth (US), while "tesla" honors an engineer with that name; note that the letter T is not capitalized when the unit name is spelled. A study of Figure 1 shows that there is a group of high numbers near the middle, and that the numbers are negative toward the upper right; however, it is difficult to see the pattern of the numbers. This pattern is clarified with the contour maps in Figure 2; each of these maps provides a different way of revealing the numbers in Figure 1. In the upper left corner of Figure 2 (panel A), lines are drawn much like those on a typical topographic map. In panel B, high, average, and negative readings are plotted as shades of white, gray, and black. The wire frame map (panel D) is excellent for seeing the peak in the numbers. The shaded relief map (panel C) is similar to this wire frame map if this bump was viewed from overhead, and it was illuminated from the upper left (northwest) side of the map. Page 2 Different styles of magnetic maps Different Styles of Magnetic Maps Each of the displays in Figure 2 has benefits and limitations. The line contour and gray scale displays are most commonly applied to magnetic maps. Line contour maps have two major advantages. The first advantage is that they allow a wide range of readings to be plotted. However, note that where the contour lines are very close together, they merge into a black area with little additional information, except that the readings are extreme. The second advantage of these contour maps is that they readily show areas where the magnetic field changes rapidly with location. This information is valuable for pairing magnetic highs with lows, and this is a fundamental part of understanding magnetic maps. The area between a paired magnetic high and low has a high lateral gradient; this is revealed by the close spacing of the contour lines. A magnetic low will usually be associated with the high toward which it has the greatest lateral gradient. If one has a printed copy of a line contour map, it may be possible to recreate the digital values that compose the map; this is seldom possible with the other styles of magnetic maps in Figure 2. Therefore, a line contour map has a greater archival value. It is generally not necessary to label contour lines with the values of the anomaly or field. This is because the actual values of the magnetic field are not too important; it is changes in the field that are important. Line contour maps can also be saved as graphics files that have a high resolution; they can be vector files, rather than raster files. The line contour maps in this report are all vector files; this allows them to be enlarged on a computer's monitor without losing resolution and the sharpness of the contour lines. If magnetic maps are not very complex, vector files can be smaller than bitmap files; on complex maps however, bitmap files will be smaller. The major disadvantage of a line contour map is the fact that it is difficult to compare readings across a wide area on a map. That is, it may be difficult to see patterns that are formed by similar readings across the width of a map; this is particularly true for large or complex magnetic maps. Gray scale maps eliminate this problem, and that is their greatest advantage. If one part of a map has a particular gray tone, then another part of the map with that same gray is caused by similar or identical magnetic readings. This continuity can be a great aid for clarifying the shapes of complex features that may be revealed in a magnetic map. The big limitation of gray scale maps is the small range of readings that can be displayed. More correctly, small amplitude anomalies cannot be displayed in those parts of the map where the surrounding values are high and also where they are low. If color can be added to a map, then a much wider range of magnetic values can be plotted faithfully. A shaded relief map, like that in panel C of Figure 2, has the advantage of familiarity, at least for someone who has seen vertical aerial photographs. These maps are called shaded, but they actually have no shadows; this is a benefit, for shadows could obscure important patterns in the maps. Dark tones in a shaded relief map mean that the surface in that area is pointed away from the direction of illumination. Shaded relief maps can accentuate linear features if the illumination is set to the correct angle; they can also Page 3 Different styles of magnetic maps attenuate these features if the illumination is in a perpendicular direction. Shaded relief maps have the disadvantage that they can increase the complexity of the patterns.