The objective of geophysical exploration from boreholes is to indicate direction and distance to promising targets. To improve the possibilities for 3-dimensional interpretation of the surroundings, a system capable of measuring three magnetic field components in the hole has been developed. The system originated in Sweden and is explained by Robert Pantze, Lennart Malmqvist and Gerhard Kristensson in their paper “Directional em Measurements in Boreholes.” The system generates a continuous wave electromagnetic field from a large ground transmitting loop, working at two frequencies: 200 and 2,000 hz. Noise is efficiently reduced by the receiver. This makes measurements in mines possible even when mining operations take place.
The probe measures the em-field parallel to the hole and the horizontal field perpendicular to the hole. The third component is perpendicular to the two other components. The three components create a right-hand system. The amplitude and phase are recorded for each component. Phase reference is transmitted to the receiver by a radio communication link. In underground work, a small loop can be used to provide a local phase reference.
Measurements have been made down to a depth of about 1,000 m. Continuous operations have been undertaken during the past three years. The potential range for the detection of a conductive target of economic size is at least 75 m.
The interpretation scheme has been developed according to two lines of thought. In general everyday work, a computerized interactive system based on graphics and simplified fast calculations is used. In addition, a more advanced interpretation system has been developed, describing all components and their phases in the presence of an infinite surface, a layered earth, and a buried, bounded inhomogeneity.
The system consists of a large ground loop transmitter and a receiver registering the three perpendicular components of the magnetic field. The transmitter can feed the loop with a 4-ampere alternating current. Its maximum power is 1,000 w. A characteristic size of the transmitting ground loop is 1,000×1,000 m.
The receiver measures the magnetic field in three perpendicular directions. The coils are placed within the 32- mm-wide probe and form a right- handed co-ordinate system. They are tuned to a band width of 150 hz. The signals from the three coils are preamplified and transmitted to the surface through a coaxial cable. A reference signal is transmitted to the receiver from the transmitter by a radio communicaton link. In the receiver, located on t he ground, the signals are processed and the amplitude and phase for the three components are computed. To improve the signal to noise ratio, the band width of the detector is reduced to 1 hz. User friendly Proton Magnetometer
Barringer Research of Rexdale, Ont., has produced an advanced magnetometer called the M-234. The device is rugged, weatherproof and lightweight and can be supplied as a base station, airborne or marine magnetometer. A variety of sensors and cable lengths are offered, including shallow marine, deep marine, base station and airborne. All sensors are interchangeable.
A unique feature of the M-234 is the menu-driven software. The menu allows the operator to easily enter or change the operating parameters in a straightforward manner. For example, to alter the cycle rate, the operator would initialize the “rate” selection of the menu. The lcd would then display the cycle rate parameter and request the operator to make a selection. In the meantime, the unit will continue to take readings at the previous entered cycle rate. Upon exiting the menu, the instrument will acknowledge the new cycle rate without interruption. The simplicity of the procedure frees the operator from thumbing through a manual or memorizing a cumbersome numerical procedure, in order to change the operating parameters. It also eliminates the use of binary switches or other awkward selection techniques. The following parameters are controlled via the menu: cycle rate, time and date, starting value for automatic tuning, line number, manual/external selection, recorder format, outputs and lcd viewing angle.
Another feature is the large area Liquid Crystal Display. The lcd displays a 6-digit magnetic field reading, supply voltage, signal strength, input values (in menu mode) and a real- time one or two trace analog representation of every reading. The lcd shows a compact analog representation of the most recent 24 readings.
The tuning is fully automatic throughout the range of the instrument. The operator also has the option to select a starting value of the magnetic field for a fast response of the automatic tuning.
Other features include: high sensitivity; wide selection of cycling rates; 4-scale selections of two overlapping scales; two recorder formats (standard and full analog); on-line single or dual trace selection; and RS232 analog and audio outputs. Insulator Links
Special insulator links which provide 50,000 volts protection are available from Miller Construction Products of Doylestown, Pa.
The links are available with eye, jaw, wedge and thimble caps, in a range of working load capacities from three to 100 tons.
Insulator links weigh from 13.5 to 620 lb, depending on the model.
Special-purpose or higher-capacity links can be manufactured to specifications. — 30 —
Be the first to comment on "Exploration: What’s New DOWN-HOLE PROBES"