Electromagnetic Fields and Gold Exploration
This gold exploration method is based on an artificial electromagnetic field set up by high frequency alternating current either passed conductively through the earth by contact or applied inductively to the ground by some kind of transmitting loop. Basically, the electromagnetic field induced is referred to as the primary field and it in turn induces secondary electromagnetic fields in any subsurface conductors such as gold ore bodies. The combined or resultant electromagnetic field consisting of the primary and any field due to the presence of subsurface conductors that are studied by means of a directional receiving unit.
It is important to indicate that electrical methods employing a vertical loops transmitter for inductively energizing the ground are the most widely employed for electromagnetic prospecting methods. Essentially, there are two basic types of instruments for this prospecting technique. In one case, one of the transmitting and receiving units are designed to function separately. A large vertical transmitting loop is operated by one person while a second guy is needed to operate the receiving unit. This equipment is suited for energizing conductive zones that are vertical or are steeply dipping such as veins, faults and dykes buries under shallow overburden. The depth range is entirely governed by the physical conditions existing at each survey, but under several and especial circumstances may reach several hundred meters.
The second type of instrument and more employed uses a method in which the transmitting and receiving units are held in a fixed position in relation to each other by means of a pair of carrying handles. This equipment has a depth range of 8 to 10 m and the measurements are influenced on the existing physical conditions and are usually operated as a single unit by one single person. Under certain types of problems the receiving and transmitting units may be separated and employed in a manner similar t that of the large vertical loop field transmitter. Nevertheless, the depth range is not increased in a great magnitude. According to some geologist, this type of unit is appropriate for locating and relative conductive body within the range of the equipment.
It has been noted that when operating the large field transmitter, the receiver is held in a horizontal position so that a minimum is registered on the receiver visual meter and in the receiver earphones. In some circumstances transverses are performed across the strike at distances from 10 to 50 meters from the receiver. In this way, indications of subsurface conductive bodies are noted in the form of increase dial tone and meter reading. Also, when an indication is detected, it is important to tilt the receiver to again obtain the null position. The extension of lines perpendicular to the plane of the receiver when so tilted will point toward the conductive body and it is possible to get information such depth and attitude of the vein or ore body as well as its plan or surface location. When using a smaller, coupled, instrument, the receiver is carefully balanced to the null position over barren ground. When the instrument is then carried over a conductive body in which a secondary field has been induced, an increase in visual meter indication is detected and also an increase in earphone tone. According to some gold prospectors, the effectiveness of work depends on the size, length, shape of the orebody, depth of the orebody. The detectability of a conductive body decreases with increasing depth.