Prospecting methods and techniques
aerial photography
Aerial photography allows you to see the geographical area and can reveal the existence of structures invisible from the ground, making it an essential tool for prospecting.
Sometimes soil alterations can be related to deposits in the subsoil; these terrain alterations go unnoticed from the mainland. By placing a grazing light that accentuates the alterations, buried structures can be observed. It is used in areas where there is vegetation or the soil is humid. In areas with vegetation it can be observed that plants grow taller where there is more land and less where there is less land, these are the so-called crop marks (described in English as crop marks).
In humid places the color of the soil varies depending on the degree of humidity, the color is darker where there is more soil and therefore greater humidity; a light color in the soil could be evidence of a buried structure.
In places without vegetation and dry, aerial photography does not offer good results.
Magnetic prospecting
It was discovered in 1959 by Aitken, it is based on the record of the local variation of the Earth's magnetic field, this variation is due to the presence in the subsoil of magnetic materials, such as iron objects, wells, etc. This technique has a high equipment cost, it also has problems of use in subsoils in which there are metallic elements that affect being registered (scrap, water conduits,...) and it cannot be carried out near electric power cables or near radio transmitter stations or on igneous rocks, such as basalt.
Geophysical prospecting
It is the most traditional technique and consists of probing the ground with bars or drills and noting the places where they encounter solid bodies or cavities. Some archaeologists still use it to estimate the depth of deposits or to explore pits. Augers are also used by geomorphologists to study the deposit's sediments. There is a risk of damaging artifacts or fragile structures.
One of the most advanced techniques has been unveiled by Professor David Thiel from Griffith University Australia and has shown significant progress.
Subsurface Remote Sensing
Probing techniques are useful but the deposit is altered, however there are a series of techniques that are not destructive. They are geophysical remote sensing devices, which involve the passage of different types of energy through the ground, based on the anomalies found by this energy, or the measurement of the intensity of the Earth's magnetic field.
Seismic and acoustic methods
The simplest way to pass energy through the ground is by striking it. Bosing (or bowsing) is the percussion of the ground with a heavy wooden mallet or a lead-filled bowl on the end of a long handle. Recording the resulting sound helps locate buried structures, as a dull thud indicates that the ground is undisturbed, and hidden ditches or holes produce a more resonant sound. This technique has become obsolete due to technological advances.
The vertical wave technique consists of a device that generates and amplifies the so-called Rayleigh waves by striking the ground gently and repeatedly. The speed of the waves can be calculated using two collection points separated by a fixed distance. Waves propagate faster in hard materials and more slowly in clay or soft materials; structures such as buried soil surfaces can be detected. The sections generated by the device can later be transformed into a contour map of the buried structures.
Radio waves and electrical impulses
Soil radar is a similar method, instead of using sound waves it uses radio waves. The transmitter emits short pulses through the ground, and the resulting echoes reflect the variations in the ground. A typical device makes readings ten centimeters long by one meter wide and a depth that can reach three meters. The readings are sent to a computer that produces a series of radiographic slices, which combined, generate a three-dimensional image of what is under the ground. Works well in dry, well-drained sandy soils. It is a slow method and is still in the experimental phase.
Georadar is another modality of this mechanism, developed by American and Swedish technicians. It consists of a larger antenna mounted on a crane arm attached to a large radar cart. It transmits electromagnetic energy to the ground that is partly reflected when it locates an interconnection of two materials or two different electrical properties. Measuring the time elapsed between the emission and reflection of the signals helps to locate the positions of different levels or buried objects. It is capable of reaching depths of four meters in peat terrain.
electrical resistivity
It is a very common method. The technique is based on the principle of "the more moisture the soil has, the less resistance it will offer to the electric current." It is very effective in ditches, chalk and gravel quarries and in clay constructions. It is very slow since four electrodes must be placed for each reading, it is also an effective complement to other remote survey methods and can replace magnetic methods since it can be used in urban areas, near power lines and metals.
Metal detector
Metal detectors are electromagnetic devices useful for locating remains under the ground. An electric current is passed through a transmitter coil that generates an alternating magnetic field. Buried objects distort the field and are detected by an electrical signal captured by a receiving coil. With them you obtain general results quickly and are of great help in locating modern metal objects. They are also used by amateurs who can discover deposits (they are the so-called "treasure hunters"). However, there are responsible professionals such as the case of researcher Daniel Vanderleven") who has been using metal detectors for several years, making innovations in their handling and detection, who has safeguarded several objects and documented more than 5 thousand classified objects. Achieving new facets of research with technology. The Montserrat sweep, the predictive advance, the Milenka technique are techniques developed in fields of battle and archeology which gives advantages in its detection, increasing the possibilities of location by 90%.
Radioactivity and neutron scattering
The use of radioactivity and neutron scattering have been experimented with in remote sensing tests, but they only work if the soil layer is very thin. Most soils have a radioactive component and in these techniques, the readings measure the discontinuity between the trenches and pits and the land surrounding them. In the neutron method, a probe with fast neutrons is introduced into the soil, which detects the slow neutrons and the ratio between them is measured. Rock generates a lower percentage than soil, so buried structures can be detected.
Thermal prospecting
It is based on the temperature change that occurs on buried structures whose thermal qualities are different from those of their environment.
Geochemical analysis
Geochemical analysis consists of taking samples of the deposit and its surroundings at regular intervals of one meter and measuring the phosphate content since these are the easiest to identify and those that give the best results. They are slow studies, but they reveal structures that other techniques do not.