Archaeologists are increasingly looking at remote sensing methods as techniques to explore sites with minimum disruption to the surroundings. This work is delivering new means of mapping prehistoric and historic sites in three dimensions rather than traditional two-dimensional methods.
Magnetics is a primary remote sensing technique that offers both ease-of-use and cost-efficiency.
The main benefits lie in the ability to resolve details non-invasively, the wide range of artifacts and cultural objects that are detectable, and the low-cost of magnetics in comparison to other methods.
Magnetometry and gradiometry resolve many structures, including buildings, cooking sites, furnaces used for smelting, burial grounds, and other types of buried subsurface objects.
Data is typically acquired using a gradiometer – a two-sensor configuration that serves to reduce natural noise from sunspot activity (diurnal effects) as well as focusing the depth of investigation to the near-surface. Depths of penetration vary up to 10m depending on the type of target being investigated (i.e. highly ferrous as opposed to weakly ferrous).
A number of case histories are available from Symetrics and GEM. Note that these case histories have been digitized and appear with less resolution as compared with the originals. However, they still provide numerous examples of the application of magnetics to archaeological investigations.
Instruments and Data Processing Overview
Several different types of instruments are available for measuring a) total magnetic field (i.e. systems from Symetrics and GEM) and b) three components of the magnetic field (i.e. fluxgate systems). Total field systems offer a number of benefits over fluxgate systems, including high rates of acquisition and no requirement to calibrate systems during surveys for greater survey efficiency.
Processing of data is straight-forward, requiring a) downloading of magnetic and gradiometric data from the instrument to a personal computer and b) minor filtering for noise suppression related to geologic or other effects not of interest to the archaeologists. Simple software packages are available for these purposes from Symetrics and GEM.
Advanced users may also be interested in applying routines, such as Analytic Signal processing to convert dipolar total field anomalies to single peak anomalies that can be easier to visualize. Other advanced routines, such as modeling to determine the depth of magnetic sources, can also be applied.
Archaeologists work in some of the most diverse terrains possible. From the world’s largest historic site at Angkor Wat to the Indigenous burial grounds in North America to the ancient Roman fortifications that cover Europe, archaeologists are “breaking new scientific ground” every day. Our magnetometer has aligned its product offerings to meet these demanding requirements with instruments that are non-intrusive to the sites under study.
It's very high sensitivity optically pumped Potassium system is capable of resolving the most subtle contrasts in materials (such as those of clay bricks in soil).
The unique Overhauser system has a wide range of “detectability” for low contrast and high contrast (ferrous) structures while matching specifications of optically pumped Cesium instruments at a much lower cost.
And where economy is required, Symetrics also offers the world’s most feature-rich Proton Precession instrument – a tool with a classic value that complements any archaeologist’s toolkit.
To give an idea, the rule of thumb is that 1 ton of steel will give 1nT at 100 ft. The distortion caused by the steel in the earth’s field falls off as the cube with distance and is linear with mass. Therefore, at 50 ft, 250 lbs will give 1nT, at 25 ft 30 lbs will give 1nT, at 12 ft 4 lbs will give 1nT. Cables and pipelines fall of at somewhat a different rate (inverse square) so can be seen further for a given mass.
The depth of exploration is determined by the spatial wave-length of the magnetic anomalies revealed during the survey. It is important to understand that the measurement of the magnetic field at a single point does do provide any depth information. A single measurement includes the contribution of the magnetic field from many sources and so it is necessary to measure the variation in magnetic field strength at many different locations in order to determine the location of the objects that are causing the magnetic field variation.
Magnetic field surveys are used during exploration for almost all types of minerals. Magnetic survey data are usually interpreted in an effort to gain a better understanding of the geologic structure beneath the survey area and this understanding is used as a guide for conducting follow-up surveys using different techniques or for choosing the optimum placement of exploratory drill holes. In addition to this, magnetometer surveys are commonly used as a means of direct detection of iron and nickel ore and also for gold exploration. In the case of gold exploration, the target mineral is magnetite which, because of its density, is used as an indicator in locating gold in fluvial deposits. Similarly, an absence of magnetite is used as indicator of possible gold deposition in epithermal conditions.
When surveying a large area or when conducting the survey of a small area over a long period of time, it is necessary to remove the back-ground variation of the Earth's magnetic field. Your engineer is correct in being concerned about the effects of magnetic storms and these are the more extreme examples of the diurnal magnetic field variation that is always present. It is customary to use a base station magnetometer to recognize and remove this variation component. We recommend our model G-856AX proton precession magnetometer for use in this role. The G-856AX is a very stable, relatively inexpensive magnetometer and includes a very stable and accurate clock, as do our models G-858 and G-859. When used as a base station, the G-856AX clock is synchronized with the survey magnetometer's clock and it is set up on a tripod and configured to run automatically in a fixed location. Each of the measurements that the G-856AX records will be automatically recorded, along with the date and time of the measurement. The same thing happens automatically for each measurement made with the roving (survey) magnetometer. Then, when the data are down-loaded to the processing computer, our MagMap2000 software can be used to process these data sets to remove the diurnal variation from the survey data and perform other data processing and analysis functions. Attached, please also find a quotation for our G-856AX with base station accessories.
Mn / Cu / Mg are not ferromagnetic and also none of the common mineral sources of these elements are ferromagnetic. So direct detection of Mn / Cu / Mg is highly unlikely. I would recommend that you retain the services of an economic geologist familiar with your prospect area to see if a magnetic survey useful in gaining understanding for the geologic structures that may be important for locating concentrations of minerals that include these elements.
1. There are basically three types of "gold": low concentration disseminated gold in ore, placer gold deposits and solid gold such as that associated with treasure. Magnetometers are used to find disseminated gold by its association with mineralized zones which also contain magnetite or other magnetic minerals. Magnetometers are often used in conjunction with airborne Electro-Magnetic surveys to find the conductive ore bodies. Placer gold is the type found in buried stream channels such as the gold which sparked the California gold-rush in 1849. Gold dust and magnetic minerals have been concentrated in river banks over thousands of years. Where there is gold there is often magnetite and therefore the magnetometer can be used to locate placer gold deposits. Gold treasure is a different story and being non-magnetic gold, silver, and other precious minerals are not directly detectable by the magnetometer.
2. The magnetometer can only detect ferrous (iron or steel) objects. If the gold is stored in an iron box or has iron materials next to the gold (such as colonial ship ballast stones in the marine environment), there is the possibility of detecting the iron material. This is true for land and marine (sunken galleon) gold bullion. The vast majority of target search surveys are performed on a grid in a "lawn mower" back and forth manner to cover the area of interest. Lane spacing is dependent on target size (magnetic mass).
3. At a sensor to target distance of 2 to 3 meters there will need to be at least 1-2 kilograms of iron. This can produce a 1-2 nT anomaly that is detectable in a magnetically clean environment. The ideal environment would be in a plowed farm field or the bottom of the ocean away from human activity i.e., away from a port or harbor. You will probably not be able to detect this small of an anomaly in a city or port location. The more iron mass there is, the better the detectability.
4. Training to use the magnetometer can take 1-2 days depending on experience with setting up computerized survey equipment and a GPS.
5. Processing the magnetic data requires several days of training and would require a geophysical background to interpret the final maps. We provide free software to make maps and estimate the target depth of burial (inversion). If you are unfamiliar with this procedure, we would recommend that you find a local geotechnical firm to look at the data to determine if there are anomalies that should be investigated further. Remembering that non-ferrous materials do not cause anomalies (gold, silver, copper, brass, aluminum, gems) you will be looking for anomalies either associated with the container OR associated with ground disturbance (i.e., gravesite). In this way some anomalies can be detected where there has been an excavation such as a gravesite.