Date of Award
Bachelor of Science (BS)
Dr. John R. Reid
The McPhar VHEM is an efficient and useful electromagnetic unit used in conjunction with geologic exploration to locate shallow-depth ground conductors which may contain ecomomic minerals.
It consists of two battery powered lightweight units, a transmitter and receiver, each with a set of two enclosed coils. It operates on an alternating current with two frequencies of 2400 cps and 600 cps.
Both vertical loop (VL) and horizontal loop (HL) procedures may be used with the same set of instruments.
With VL, two techniques, VL Broadside and VL Standard are used; VL Broadside for locating the anomaly, and VL Standard for tracing out its length. The process is to pass the instruments parallel over the conductor and thus obtain a VL profile curving from negative to positive with the crossover at zero over the conductor.
The basic principle of VL is that an induced current sets up a secondary magnetic field around any conductor within range. The receiver detects the dip of the resultant vector of the primary and secondary fields so produced.
HL procedure is primarily used to determine the nature of the anomaly. With this method the receiver follows the transmitter in line and perpendicularly across the conductor.
The principle of HL is to measure the secondary field relative to the primary field. The instruments are interconnected by a cable which allows the required voltage to balance out the secondary field. This voltage is recorded as percent of normal voltage. When the in-phase (IP) readings are plotted, generally a symmetrical profile results with two positive shoulders and a large negative central lobe. The out-of-phase (OP) curve tends to be rather flat, especially with good conductors.
From the HL profiles several interpretations can be made. High IP/OP ratios indicate good conductivity. The profile width between zero readings denotes conductor width. Asymmetry generally indicates dip.
The readings after any necessary corrections are generally plotted on graph paper either as individual profiles or as a composite profile plan map.
The main error with VL is transmitter disorientation with respect to the receiver position. This results from variance in sound direction caused by rough topography between the operators. The error may be corrected by running both instruments on measured lines, by decreasing separation between instruments or by a pace and compass method. Disorientation error can be critical with weak and/or complex anomalies.
Some error with the HL procedure may be caused by static or instrument noise due to poor insulation and grounding of the various instrument components. However, HL error mainly results from short cable effects and differences in the elevation between the instruments. Short cable error can be best corrected by adjusting for it directly in the field with the help of a correction chart.
Also, error due to elevation differences between the instruments can be eliminated in the field by always setting the coils of both instruments parallel to each other when taking HL readings.
Certain types of complex weak conductors can cause problems in the field with VL Standard. These types include irregular broad strongly magnetic zones; crossing conductors in jounts, fractures and/or faults; warped, curved conductors; and intermittent conductors. To alleviate difficulties in pinpointing these conductors a decrease in instrument separation is recommended.
The essential criteria of a good conductor is that the conducting material be continuous. Pyrrhotite and graphite are usually excellent conductors. The conductivity of other sulfides may vary from negligible to good, depending upon the type of environment in which the anomaly lies.
Accurate interpretation of VL profiles is usually difficult due to the large number of variables which may be involved in the conduction and the high probability of at least some error being introduced into the readings. However, controlled model and field studies are very useful for defining the conditions for particular trends.
HL profiles can be used to interpret conductivity, width and approximate dip and depth of burial. Nipping conductors cause asymmetry in a profile. A high central lobe results if the traverse is at an acute angle to the conductor or, if conductor is wide. Magnetite promotes high profile shoulders. A high IP/OP ratio indicates a good conductor.
Lassila, Pentti, "The McPhar Vertical and Horizontal Loop Electromagnetic (VHEM) Unit as a tool in Geological Exploration" (1968). Undergraduate Theses and Senior Projects. 55.