Airborne Magnetometer (1952)

prospecting for minerals and oil is a time-consuming and exacting task but today thanks to an airborne device that had its origin during World War two it is possible to extend and speed up considerably the task of prospecting to understand why and how this airborne device is used in prospecting let us first briefly review something about magnetism here is a bar magnet surrounding this magnet there exists a magnetic field that is to say a space in which the magnet exerts a magnetic influence some idea the shape and strength of this magnetic field can be demonstrated by means of a simple experiment with a bar magnet and iron filings notice how the iron filings arrange themselves into a pattern of lines running from one end of the magnet to the other notice too that these lines are bunched more closely together at the ends of the magnet here is where the strength of a magnetic field is greatest in other words these lines known as magnetic lines of force indicate by their shape the direction of the magnetic field and by their concentration the relative strength or intensity of the field now let us see what happens to a magnetic field when we introduce into it some substance for example iron no longer is the magnetic field symmetrical in the region surrounding the iron we see from the closer bunching of the magnetic lines of force that here the strength of a field has increased here is another magnetic field on the irregularities present in this field can you find the substances hidden from view when this technique highly refined of course is applied to the earth which acts very much like a bar magnet it affords an excellent tool for prospecting in the case of the earth irregularities in its magnetic field can be detected by this airborne device popularly referred to as the bird the bird is the detecting unit of the overall equipment known as the airborne magnetometer let us examine this bird more closely this is the head of the bird notice that it is free to move in any direction embedded inside this plastic head section are three small coils which serve as the brain of the magnetometer the center one which looks like this detects the Earth's total magnetic intensity at any point in space the other two coils are set at right angles to the center coil or detector they operate as a team to orient the detector so that it always parallels the Earth's magnetic line of force watch how they do this another part of the magnetometer is this recording device which makes a continuous record of variations in intensity of the Earth's magnetic field picked up by the detector as the bird is towed through the air the airborne magnetometer is extremely sensitive and the influence on the Earth's magnetic field of even a small screwdriver is easily detected now let us join a Geological Survey crew and see how the airborne magnetometer is put to work normally a narrow magnetic survey crew consists of four men a magnetometer operator who is also an experienced geophysicist an observer a pilot and a co-pilot prior to each flight the pilot visits the local CAA communication station or weather bureau to check the weather there is no point going aloft when you cannot see the ground from the altitude selected for the survey when necessary the pilot also obtains a waiver of the civil air regulations for the flight on the local CAA aviation safety agent for each area to be surveyed the crew is supplied with a series of aerial photographs and observer maps on which is indicated the flight pattern to be flown generally the most efficient flight pattern for covering the project area is a series of parallel flight paths or traverses flown at right angles to the terrains major geologic trends spacing of the traverses depends upon the geology of the terrain and normally varies from one quarter to one mile near the ends of a parallel traverses and at right angles to them to additional flight paths or base lines are flown these are flown first in one direction and then immediately in the other the other flight traverses are flown in only one direction traverses of not more than 25 to 50 miles are normally selected since longer ones would be unduly fatiguing to the pilot these traverses are also numbered for quick identification well we are about ready to start let us get aboard the plane as soon as the plane is airborne the magnetometer operator starts the Dyna motor which converts the planes electric power to the proper voltage and frequency needed to operate the magnetometer and associated equipment he also turns on the inverter for the camera gyro important to the success of every arrow magnetic survey is of course the pilot to do his job satisfactorily it is necessary that he be able to interpret aerial charts and photographs rapidly and that he have had considerable experience in photo mapping and low-level flight work such as crop dusting when the plane reaches a position about 15 to 20 minutes from the project area the copilot advises the others in the crew to lower the bird this operation requires the services of two men and must be done very carefully the insulated cable supporting the bird is played out slowly until it attains a length of about 80 feet at this distance metallic substances in the plane have no appreciable effect on the magnetometer this is true of the cable itself since its core is made of phosphor bronze a non-magnetic substance with the bird released each man proceeds to his next task to the observer is assigned the task of readying the aerial camera which makes a continuous photographic record of the terrain during each flight traverse the photographic image recorded by this camera is corrected for the motion of the plane in flight by a system of gyro stabilized mirrors after the observer loads the camera with film he takes a reading of the intensity of the outside light with this information plus the altitude and speed at which the plane will be flown over the project area he selects the appropriate film speed and slit aperture settings and adjust the camera controls accordingly the magnetometer operator meanwhile is busy calibrating the magnetometer recording equipment he does this at the beginning and end of each day's operation changing the settings of these controls changes the value of each division of the recording tape and the base level of its scale it is the magnetometer operators job to so select and set these controls as to produce an uninterrupted record no matter how greatly the magnetic intensity x' may vary this is the sensitivity control it permits setting a scale division to read any value from 2 to 100 gammas the unit in which magnetic field intensity is measured the other three knobs are selector controls they control the value of the scales base level the first one changes it by multiples of 5000 gammas the second one by 500 and the third by 50 each control is geared to a Stamper which is used to record the control settings on the tape for future compilation the digits in this number refer to the settings of the sensitivity control and the 5550 gamma selector controls there's 4 digit number of which we will learn more later is used to identify and coordinate the magnetic record with respect to landmarks elected by the observer this mark here called a fiducial is used to represent a selected landmark each fiducial as an identifying number and this same number appears on the observers map and photographic record to identify the same landmark the magnetometer operator also checks the operation of the radio altimeter recording equipment this equipment makes a continuous record of the plane's altitude during each flight Traverse port is important in the compilation of the magnetic data to know how much if any the plane varied from its assigned altitude fiducial marks on this chart also represents selected landmarks and are recorded simultaneously with those reproduced on the magnetic and photographic records well everything seems to be working ok here let us see how the observer is coming along he is now checking the mechanism which simultaneously produces fiducial marks on the magnetic altitude and photographic records and at the same time actuates the electric number cutters each time he depresses this button the number changes here at the magnetometer operators hand stamp and in addition causes a photographic record to be made of this same number as revealed by a counter built inside the aerial camera the observer also readies the especially designed and constructed drift meter used for scanning the terrain along each Traverse the drift meter has an extremely wide angle lens which permits a scan of close to 180 degrees as the plane nears the project area the pilot reduces its altitude and speed to that assigned for the survey in this instance 140 miles per hour at an altitude of 500 feet the pilot and co-pilot now give their attention to the aerial photographs on which are drawn the paths to be flown both note carefully the appearance of the terrain along the first traverse and then scan the horizon for these same features as soon as the spot marking the start of the first traverse is cited the product sets his planes course to cross identifiable points along the Traverse while the co-pilot warns the other crewmembers that the first run is about to begin immediately upon hearing this the magnetometer operator quickly aligns and uncage --is the camera's gyro and starts the camera meanwhile the observer scans the terrain for the starting point when the plane is directly over it EDA presses this button twice in rapid succession and then marks and numbers the spots approximate location on his map as the double fiducial mark denoting the start of the run appears on the magnetic record the magnetometer operator identifies it by hand stamping the tape he also numbers the starting point on the altitude record incidentally the altitude record runs at a much slower speed than the magnetic tape a constant watch of the recording equipment is maintained during each Traverse and when necessary its controls are reset to prevent the stylus from running off the tape the values of each new setting are always immediately hand stamped on the tape the observer continues to scan the terrain each time the plane passes over an easily identified spot he depresses the button to mark its location on the photographic magnetic and altitude records its approximate location and identifying number are also noted on his map in addition he keeps a log of pertinent actions performed during each flight traverse piloting skill of the highest order is needed to follow exactly each Traverse at the required speed and altitude when the end of the Traverse is about to be reached the co-pilot again advises the others however the pilot does not start his turn to the next Traverse until the observer double punches the button to mark the end of the run okay to start the turn now during the turn which normally takes two minutes the recording equipment continues to operate after each turn and prior to the start of a run the magnetometer operator rejects the cameras gyro to be sure it is properly aligned if necessary it is caged realigned and then uncaged here we go again this time flying south along Traverse number two this process is continued until each Traverse scheduled for the day is flown normally an average of 500 miles of traverses can be covered in a day this represents an increase of as much as 100 times the amount that could be surveyed by ground methods furthermore the airborne magnetometer is considerably cheaper to operate on a per mile basis than a similar ground survey and even more important it is usable over terrain unsuitable for ground exploration actually arrow magnetic survey is only one of many important scientific and technical applications of aircraft being made today let us see now what is done with the survey data after it is collected from the information obtained by the airborne magnetometer a narrow magnetic map is prepared the preparation of a narrow magnetic map is in many respects similar to that of a topographic map but here the contour lines represent magnetic values and are drawn so as to connect magnetic values which are equal in its completed form the arrow magnetic map looks like this a series of magnetic contour lines superimposed on a geologic map of the area surveyed where the contour lines indicate a peak such as here a magnetic anomaly is present thus it is not surprising to learn that iron ore deposit exists in this area these black dots represent its location here is another peak or magnetic anomaly and close by another iron ore deposit actually the location of mineral deposits is not as simple as it appears here nevertheless the arrow magnetic map when interpreted and used by skilled geologists and geophysicists does provide the quickest cheapest and most effective technique for localizing the areas which merit more detailed examination by slower and more expensive geological or geophysical methods arrow magnetic surveys by the United States Geological Survey have already been conducted in more than 30 states and plans have been completed to extend this work into many more the United States Geological Survey is convinced that in the search for many kinds of mineral deposits needed for our own and the world's ever-expanding industrial economy the airborne magnetometer is destined to assume an increasingly important role you