BACKGROUND
Like gravity surveying, magnetic surveys are based on the premise that a target is limited in space and has a different physical property, in this case magnetic susceptibility, from the surrounding geology. Unlike gravity surveying, however, the variation in magnetic susceptibility for various rock types is orders of magnitude greater than the variation in density for the same rock types. Thus, even knowing the types of rocks in a specific area does not provide sufficient information to constrain susceptibilities. Like density contrast, variations in susceptibility tradeoff strongly with other model parameters. Therefore, if susceptibility, or other model parameters, can not be constrained from different observations, it is difficult to make quantitative estimates of the geologic structure based on magnetic observations alone.In this particular survey, we do have the additional constraints that allow us to use the magnetic observations in a quantitative fashion. This information is derived from two separate data sets: geology and gravity. Therefore, the procedure we will use in designing the magnetic survey is to first constrain our geological model using the gravity observations collected previously. Once we have constrained the range of plausible geological models from the gravity observations along the line corresponding to y=0, we will design a magnetic survey to estimate the spatial extent of the structure producing the observed gravity anomaly and estimate its susceptibility.
In planning the magnetic survey, we will follow the same procedure used in planning the gravity survey. We will predict the noise from sources not of interest in the survey, estimate the standard deviation of the random (operator and instrument) noise, calculate the shape of the signal (the theoretical anomaly produced by the assumed source), then decide whether the noise can be reduced to the point where the signal will be interpretable. If the answer is affirmative, then we determine the survey parameters that will produce the best compromise between cost and data quality.
OBJECTIVES
- Begin to understand the power of using multiple geophysical methods in an integrated geophysical survey,
- Develop a conceptual understanding of the shape of the magnetic anomaly associated with a particular geologic target, and the sources of noise that mask that anomaly,
- Learn to codify the decision-making process and to quantify conclusions,
- Reinforce the fact that economics is a part of all engineering practices.
- Reaching a conclusion on survey parameters (for our use in creating synthetic data) and
- Submitting a bid that takes into account both economic and technical factors.
PROCEDURE
Using information provided in the Request for Bid and the geological overview, do the following:- Create a suite of geological models that may be responsible for the gravity anomaly observed on west end of the line,
- For each geological model in this suite, construct a geophysical model of the structure. That is, given the geology, how do geophysically relevant parameters, in this case density since we will be attempting to model a gravity anomaly, change with depth or position. For example, vertical faults and sills could be represented geophysically by thin horizontal slabs of more dense material. Notice that a simple geophysical model could represent multiple geological structures. Thus, it is not possible from the geophysical model alone to infer geological structure. Also, notice that most, if not all, of these geological models can be represented to first order by relatively simple geophysical models: slabs, cylinders, spheres, etc., and
- To test whether any of these geophysical models can explain the gravity observations, download the appropriate modeling scripts pointed to below and attempt to model the gravity anomaly observed on the western end of the line.Slab ModelsCylinder ModelsSphere Models
- These scripts should be adequate for you to model the reduced gravity observations in terms of a variety of geological models. From these models, determine which could produce the observed gravity anomaly with geologically plausible parameters. Using the geological information you can constrain the range of plausible density contrasts for each model. Is it possible to model the gravity anomaly within this range of density contrasts? If not, then the geological model can not plausible explain the geophysical observations.
- Once you have limited the range of plausible geological models, use the gravity observations to constrain the geometry (depth, width, position, etc.) of the density anomaly producing the observed gravity anomaly.
- Now that you have constrained the geophysical model, run the script pointed to below and produce a series of plots of the magnetic anomaly associated with your geophysical model. These plots should include not only variations in geometry, but also allow susceptibility to vary. Note that for the magnetic anomaly, the geometry includes its trend direction.
Magnetic Model - Develop a detailed plan for a survey that can be expected to acquire data sufficient for interpretation of the actual geological target. Some technical issues to consider include:
- Amplitude of the magnetic anomaly
- Width of the magnetic anomaly
- Standard deviation of the random noise
- Elimination of temporally and spatially coherent noise
- Estimate the cost of the survey as designed above, then consider whether the survey design can be modified to reduce cost without causing significant degradation of data quality. Economic factors* governing the survey include:
- It takes 30 seconds to take a reading
- Mobilization and demobilization will require 1/2 day each
- Total person-hours required for processing, interpretation and report preparation is the same as total person-hours in the field
- Estimate the diurnal component of the field by continuously monitoring the field at a base station. Thus, you will need to rent two instruments and supply field crews for both.
- Field hands make $10/hour, and two are required at all times in the field with the survey instrument. One is required at all times to monitor the base station instrument.
- Field hands will only work 8 hours per day.
- Processors, interpreters and report writers make $20/hour
- Subsistence and travel expenses are $100/person/8-hour day while doing the field work
- The magnetometers depreciated at the rate of 1%/day (original cost = $7,500)
- Vehicle depreciation is $50/day
- Fringe benefits for employees are 25% of salary
- Overhead is 100% of total direct cost excluding equipment depreciation
- Profit is ---your choice---
- My consulting fee is $200/hour
OUTCOMES
Detecting Unexploded Ordnance (UXO)...
Unexploded ordnance exists at current and former sites where the military has used an area for target practice or other activities. The ordnance range from 20 mm projectiles to air-dropped bombs having weights of 454 or 907 kg. Depths of burial range from a few centimeters to a several meters for the air-dropped bombs. However, the vast majority of items are found to lie within a meter of the ground surface.
These areas are usually large and require a sustained and methodical approach to adequately cover the area. If the area is spatially fairly small, then a different approach may be warranted. The techniques used to search for UXO are mostly magnetic and electromagnetic methods. Both ground and airborne magnetic and electromagnetic systems are used. Other techniques used on a more limited extent include Ground Penetrating Radar (GPR) and airborne Synthetic Aperture Radar (SAR).
Positioning of the data is very important for UXO surveys, especially where a survey is completed and then an excavation teams return later to excavate and remove the ordnance. Positioning is usually done using high precision Global Positioning System (GPS) where centimeter-level accuracy can be obtained under ideal conditions.
Table 9 shows the depths for various ordnance at which electromagnetic and magnetic methods can detect them.
In addition to the geophysical methods mentioned above, there are other techniques based more on physics than geophysics. These methods will be mentioned for completeness but will not be discussed in detail. Each of the geophysical methods used for UXO exploration will be discussed below.
Table1. Ordnance penetration and detection using magnetic and electromagnetic methods.
Methods
Basic Concept: Magnetic methods are appropriate for UXO composed of or including ferromagnetic materials. The Earth's magnetic field induces a secondary magnetic field in ferromagnetic objects. This secondary field distorts the Earth's magnetic field around the object. Magnetometers measure the intensity of the total magnetic field, which is about 35,000 nT (nano Tesla) at the equator and 60,000 nT at the magnetic poles. Distortions to this field may be only fractions of one nT for small ordnance to tens or hundreds of nT for large ordnance. The anomaly magnitude decreases with distance from its source. The decrease depends on the size of the object relative to the height of the sensor, but for a small object, will decrease as the inverse square of the distance from the source (ordnance). Thus, the amplitude of an anomaly decreases rapidly with distance from the source. The Earth's magnetic field intensity is continuously changing, usually by only a few nT per hour, often called the Diurnal (daily) drift. However, sometimes the field intensity can change by tens or hundreds of nT per hour during magnetic storms. This effect has to be accounted for during processing of the magnetic data.
There are numerous kinds of magnetometers including cesium, potassium, proton precession, fluxgate, fiber optic, and Superconducting Quantum Interference Device (SQUID). Probably the most common types in use for UXO search are the cesium and potassium magnetometers. Figure 1 shows a cesium magnetometer with a GPS system for positioning.
Figure 1. Cesium magnetometer with GPS. (Geometrics, Inc.)
Figure 2. Gradiometer using two cesium sensors. (Geometrics, Inc.)
In addition to measuring the total magnetic field strength using one sensor, the gradient of the magnetic field is also measured using two sensors spaced a nominal distance apart. This instrument is called a gradiometer. Figure 2 shows a gradiometer using two cesium magnetometer heads. Since the sensors are arranged one above the other vertically, the instrument would measure the vertical gradient if used as illustrated.
Measuring the vertical magnetic gradient has two advantages. The first is that the output data are much less affected by diurnal drift, and the second is that the vertical gradient peaks over the top of a magnetic source whereas the total magnetic field anomaly does not. Figure 3 shows the total magnetic field and vertical gradient from a spherical object magnetized by the Earth's field. However, the depth of investigation of the vertical gradient method is less than that with the total magnetic field.
Figure 3. Total magnetic field strength and vertical gradient anomalies.
In addition to hand-held magnetometers, such as those discussed above, magnetic sensors are now assembled on frames with wheels. One such system is shown in figure 4.
Figure 4. The MTADS UXO detection system. (Blackhawk GeoServices, Inc.)
MTADS is an acronym for Multi sensor Towed Array Detection System. The sensor array is mounted on a non-magnetic structure and is towed by a special vehicle constructed to minimize its magnetic field. The system includes GPS allowing positioning to within a few centimeters. The system can be configured to measure either the magnetic field, the vertical magnetic gradient or with electromagnetic sensors. An on board computer controls the data acquisition and stores the data in memory. A similar system is called Surface Towed Ordnance Locator System (STOLS) and is shown in figure 5.
Figure 5. STOLS UXO detection system. (Geocenters, Inc.)
In addition to the above equipment, all of which stores data in memory for later downloading and processing, there are hand-held instruments that do not store the data and are used to locate anomalies that are marked in the field during the survey. One of these instruments is called a Schonstedt gradiometer. One gradiometer manufactured by the Schonstedt is shown in figure 6.
Figure 6. Magnetic location instrument. (Schonstedt Instument Company)
Data Acquisition: Magnetometer surveys for UXO must provide coverage of the area of interest with as near to 100% subsurface coverage as possible. Data is normally recorded along lines crossing the area. Surveyors paint markings are sometimes used to mark the lines that have been surveyed.
Prior to data recording, the instruments are checked each morning to ascertain that they are functioning properly. In addition, the GPS system is checked. Tests are conducted to determine the amount of time between the GPS sensor taking a reading and the reading being logged with a time stamp in the computer, called the latency of the system. Latency also takes into account the time interval between GPS readings. With the MTADS system, when recording data traveling a few kilometers per hour and attempting to position anomalies to within a few centimeters, latency is an important correction to apply. Latency is less of a problem for hand-held magnetic surveys, where data are recorded at walking speed.
A magnetic base station must also be set up to record the diurnal variation in the Earth's magnetic field. This instrument and the magnetometer recording the field data must be time synchronized so that the time when the diurnal field is measured by the base station corresponds to the time the data are recorded by the field magnetometer. Magnetic data are also collected using airborne surveys. Figure 7 shows a helicopter-mounted magnetometer system.
Helicopter surveys require special navigation aids so that the area of interest is covered. Lines are pre-planned, and the navigation system then informs the pilot of the correct course to take.
Figure 7. Helicopter-mounted magnetometer system. (Furgo Airborne Surveys)
Data Processing: Data processing involves assigning the correct spatial coordinates to each magnetic field reading and correcting the magnetic field data for diurnal variations.
Data Interpretation: Anomalies from magnetic field data consist of a magnetic high to the south of the magnetic source and a magnetic low to the north of the source. Neither the magnetic high nor the low are positioned over the top of the source, making determination of the spatial coordinates of the source difficult to locate accurately. Additional processing can be done to make the anomaly peak over the top of the source. This process is called calculation of the analytic signal and is routinely done for many magnetic surveys. The analytic signal and total magnetic field are shown in figure 8.
Figure 8. Analytic Signal
Figures 9 and 10 present the total magnetic field for an airborne helicopter magnetic survey (figure 9) and the analytic signal calculated using the same data. These data were acquired while flying at a height of a few meters above the vegetation, with some of the data being obtained at elevations of about 6 meters above the ground surface, over an area where earlier military activity had occurred.
Figure 9 shows the positive (red) and negative (blue) magnetic field values associated with each magnetic source. In figure 10 showing the Analytic Signal, only positive anomalies are seen. The peak of these anomalies is positioned over the source of the anomaly. Depths to the source of the magnetic anomalies can be found from the analytic signal.
Figure 9. Total magnetic field data from a helicopter magnetic survey
Figure 10. Analytic signal for data shown in figure 9.
Advantages: The magnetic method is fairly easy to use and provides and anomalies from both near surface UXO and those buried at some depth, depending on their size.
Limitations: The magnetic method detects anomalies only from ferromagnetic sources. Objects made from stainless steel, copper, or aluminum will not provide a magnetic anomaly. If the diurnal variations in the Earth's magnetic field are high, it may be difficult to remove its influence completely from the data. Interpretation of the data for detailed analysis of the source of the anomalies is difficult. Thus, it is usually not possible to obtain any of the parameters (length, diameter) of the source of the anomaly.
Basic Concept: Electromagnetic methods use actively created electromagnetic fields to excite metal objects. These objects then radiate their own electromagnetic field that can be detected.
Two kinds of electromagnetic instruments are available. One operates in the frequency domain, and the other operates in the time domain. These instruments have a transmitter and receiver coil. The transmitter coil generates the electromagnetic field, and the receiver coil measures the resulting signal. Instruments that operate in the frequency domain transmit an electromagnetic field whose amplitude is sinusoidal at one or several frequencies from 30 Hz to over 20 kHz. Instruments that operate in the time domain generate electromagnetic pulses having a square wave shape for the signal. Electromagnetic systems can be used on the ground or they can be attached to aircraft and used for airborne surveys. Airborne EM systems have been used for mineral exploration and environmental surveys for many years. However, although one company (Fugro) advertises airborne EM systems for UXO, no data could be found regarding this system. Geonics Ltd. and the U.S. Army Corps of Engineers are designing another system.
There are several ground EM systems that are being used for UXO exploration. These include the EM61 (Geonics Ltd.) and GSM3 (Geophex, Inc.), all of which record data to memory. Systems that do not record data to memory include the Fisher and a metal detector manufactured by Minelab of South Australia and marketed by Geometrics under the name Metal Mapper.
The EM31 (figure 10) is primarily designed to measure the electrical conductivity of the ground. It does record an in-phase component reported to be appropriate for detecting metal, and tests have been conducted to find its usefulness for UXO exploration. As might be expected, the results were not encouraging since only large pieces of metal can be detected, and the instrument is rarely, if ever, used for UXO detection.
Figure 11. The EM31-MK2 instrument. (Geonics, Ltd.)
Probably the most commonly used instrument for UXO exploration is the EM61. This instrument is used as a single instrument, or it can be configured as a set of three, allowing greater ground coverage along each traverse.
Figure 12 shows a single EM61 being manually towed. The backpack contains controls and batteries. This system uses two coils, one placed vertically over the other with their planes parallel. The lower coil contains both a transmitter and a receiver coil and the upper coil is just a receiver coil. Data are recorded using both the lower and upper receiver coils. Since the lower coil is more influenced by surface metal than the upper coil, data from these two coils can be used to reduce the influence of surface metal on the data.
Figure 12. The EM61-Mk2 instrument. (Geonics, Ltd.)
Figure 13 shows a system with an assembly of two EM61 instruments being towed by an All Terrain Vehicle. A GPS antenna is placed over the central EM61 to position the data.
Figure 13. An array of two EM61 instruments. (Blackhawk GeoServices, Inc.)
Figure 14. The GEM3. (Geophex Inc.)
Hand-held instruments include the Metal Mapper (Geometrics), the GEM3 (Geophex), the EM61 hand-held version, and the Fisher electromagnetic metal detector. Figure 14 shows the GEM3; Figure 15 shows the hand-held version of the EM61.
Figure 15. The EM61-HH2 hand held instruments (Geonics, Ltd.)
The GEM3 shown above has a GPS system attached allowing the position of the sensor coils to be determined at all times.
Data Acquisition: Surveys conducted with the towed systems are done along lines crossing the area of interest. Surveyor's spray paint may be used to show which lines have been recorded. At the start of each day, the equipment is tested over a known target to check that it is functioning properly. As with the magnetic surveys, latency tests with the GPS systems are required.
Hand-held instruments are usually used for "real time" surveys where the anomalies are located and marked at the same time.
Data Processing: Data from a towed array system, where a GPS positioning system is used, has to be processed and then applied to the electromagnetic data so as to provide the coordinates of the recorded data. The EM61 data can be processed using data from the two receiver coils. This allows a "differential" channel to be produced. This procedure involves amplifying the data from the upper coil so that anomalies from near-surface metal are seen as anomalies with similar amplitudes on both coils. The difference is then found between the data from the two coils, allowing the near-surface metal to be screened out of the data. During processing, the depth to the source of the anomaly is also calculated.
Data Interpretation: Electromagnetic data from UXO anomalies can theoretically be modeled to provide information regarding the shape of the source. Although this is not commonly done at the current time, it does have the potential to assist in preliminary discrimination since long objects, which might be more likely to be UXO than round objects, can be identified.
Advantages: The electromagnetic method reliably detects buried UXO and is not influences by diurnal variations as is magnetic data.
Limitations: The resolution for the location of the source of any anomaly is diluted slightly by the area of the transmitter and receiver coils. This may be somewhat more significant with the EM61, which has coils with quite a large surface area, compared to the hand held EM61, which uses a much smaller coil. However, the size of the coil and the electrical current passing through it and used to produce the electromagnetic field are strongly related to the depth of investigation of the instrument. Exploration depths with the EM61 are limited to about 3 m.
Basic Concept: Ground Penetrating Radar (GPR) can be used to locate UXO. However, generally the method is slow and rather cumbersome compared to the magnetic and electromagnetic methods. It is primarily used for surveys covering small areas.
The depth of investigation of GPR depends on the soil conditions. A saturated soil with significant clay will severely limit the penetration of the method. Ideal conditions are unsaturated, clay free soils. Depth of penetration also depends on the frequency of the signal. Different antennae provide different frequencies, which range from about 25 MHz to 1500 MHz. A low-frequency antenna (signal) will provide the best penetration depth. For UXO detection, a high-frequency antenna will be needed to locate small ordnance. A lower frequency antenna can be used for larger ordnance.
The GPR instrument consists of a recorder and a transmitting and receiving antenna. Figure 16 provides a drawing illustrating the GPR system.
Figure 16. Ground Penetrating Radar system.
Figure 15 shows the GPR signal being transmitted into the ground. When it reaches an object, or interface with different dielectric properties, part of the wave is reflected back to the ground surface, where it is recorded by the receiving antenna.
Several companies manufacture GPR equipment. These include Geophysical Survey Systems, Inc (GSSI), GeoRadar, Inc, Mala GeoScience, and Sensors and Software Inc.
The 100 MHz antenna is suited for deeper applications to depths of about 20 m. This antenna can be used to locate larger ordnance at depth.
Data Acquisition: GPR surveys are conducted by pulling the antenna across the ground surface at a normal walking pace. The recorder stores the data as well as presenting a picture of the recorded data on a screen.
Data processing: It is possible to process the data, much like the processing done on single channel reflection seismic data. Processing can include distance normalization, vertical and horizontal filtering, velocity corrections, and migration. However, depending on the data quality, this may not be done since the field records may be all that is needed to detect the ordnance.
Data Interpretation: If the depth to an anomaly is required, the speed of the GPR signal in the soil at the site has to be obtained. This can be estimated from ts showing speeds for typical soil types, or it can be obtained in the field by conducting a small traverse across a buried feature whose depth is known. Probably the most important feature of GPR is its ability to detect both metal and plastic UXO. Neither magnetic nor electromagnetic methods can detect plastic objects.
An example of GPR data from a site in Arizona is shown in figure 17, illustrating anomalies from both metal and plastic targets.
Figure 17. Ground Penetrating Radar data across buried metal and plastic mines. (Powers, et al. 1996)
Advantages: Probably the biggest advantage is that this method can detect both metal and plastic (non metal) buried UXO.
Limitations: Probably the most limiting factor for GPR surveys is that their success is very site specific and depends on having a contrast in the dielectric properties of the target compared to the host overburden, along with sufficient depth penetration to reach the target. However, it is likely that most ordnance will provide the desired dielectric contrast needed. Thus, depth of penetration and the resolution required are probably the most important factors. In soils containing many cobbles, reflections may be obtained from these cobbles making it difficult to distinguish cobbles from UXO. Another disadvantage of GPR systems is that the electromagnetic field transmitted could potentially detonate ordnance with electric fuses; therefore, its use is prohibited in areas where these fuses may exist.
Synthetic Aperture Radar
Two basic radar systems are used for various applications: real aperture radar and Synthetic Aperture Radar (SAR). The difference between these two systems is that SAR uses extensive processing to increase the effective size of the antenna. This results in higher resolution in the "along track" direction. SAR is predominantly used as an airborne system.
Other Methods
The following is a partial list of other methods used to locate UXO. These are either borderline geophysical methods or are physics-based rather than geophysics-based.
- Airborne GPR
- Infrared Radiometry
- Light Detect and Ranging (LIDAR)
- Acoustics. This technique is probably best suited to underwater detection.
.
General Introduction to Airborne Magnetic Surveys
During the past fifty years a very large number of both ground and airborne geophysical techniques have been developed to assist in mineral and hydrocarbon exploration. Airborne methods are usually the most cost effective tools available for both large regional reconnaissance surveys used as aids in geological mapping and for locating target areas for more detailed follow-up using helicopter borne instruments. Ground techniques are usually most effective when used to test targets discovered by the airborne surveys. In this workshop we will limit our discussion to airborne methods and only mention ground techniques in passing. We will concentrate on four of the most common types of aerial surveys:
- Total magnetic intensity surveys;
- Vertical magnetic gradiometry surveys;
- Helicopter borne frequency domain electromagnetic surveys or HEM surveys;
- Radiometric surveys.
Because aeromagnetic surveys are probably the most common type of airborne geophysical surveys, we will use this survey type as the vehicle for a discussion of field practice and many other elements of airborne geophysical surveying. However, many of the practices discussed for airborne magnetic surveys also apply to all other types of airborne surveying.
1.1 Survey Costs
It is difficult to predict the exact cost of conducting airborne surveys. Parameters that will influence the cost include:- survey type
- number of parameters measured
- number of delivery products required
- type of platform required (helicopter or fixed wing)
- measurement and navigation tolerances acceptable
- time of the survey
- location
- current fuel costs
- ferry time required
- other required products - e.g. interpretation products
- survey size.
2. Airborne Magnetometer Surveys
Aeromagnetic surveying is probably the most common airborne survey type conducted for both mineral and hydrocarbon exploration. Because total magnetic intensity and vertical magnetic gradient surveys are intimately related (total intensity data is always collected simultaneously with vertical gradient data) we will discuss both methods in this section, The type of aeromagnetic survey specifications, instrumentation, and interpretation procedures, will depend on the objective of the survey. Generally, we divide aeromagnetic surveys into two classes: regional and detailed surveys.Regional surveys usually have a relatively wide traverse line spacing, 500 meters or more, and cover an area of at least 5,000 square kilometers. This class of survey is usually done for one of the following purposes:- Geological Mapping to aid in mapping lithology and structure in both hard rock environments and for mapping basement lithology and structure in sedimentary basins or for regional tectonic studies.
- Depth To Basement mapping for applications to petroleum, coal and other non metallic exploration in sedimentary basins or mineralization associated with the basement surface such as strata bound Pb-Zn deposits or U-bearing basal pebble conglomerates.
Detailed surveys have a line spacing of less that 500 meters and are done for a variety of reasons, usually in conjunction with other airborne methods. Reasons for conducting a detailed survey include:
- Direct prospecting for magnetic ores like magnetic iron ores, crome, asbestos-bearing ultramafic rocks, or kimberlites.
- Indirect prospecting, in combination with other methods or alone to:
- Discriminate between metallic and non-metallic conductors.
- Assist in interpreting body geometry and depth
- Determine the geologic environment of the source.
- Locate specific "basement targets" for investigation using seismic methods in deep hydrocarbon exploration.
- Define the "regional" field for gravity interpretation in sedimentary basins.
- Map weak magnetic lineations related to faulting within the sedimentary section in some hydrocarbon plays.
2.1 Basic Principles
Magnetometer surveys map local disturbances in the earth's magnetic field that are caused by magnetic minerals in the upper regions of the earth's crust. If no magnetic minerals were present in the crust the earth's magnetic field would be very smooth because it originates within the earth's core and therefore comes from a very great depth. This very broad, smooth magnetic field, the "geomagnetic field", is present in all survey data even though it usually cannot be perceived within the limited areas involved in contour maps of magnetically active regions like shield or many mountainous environments. It is a slow but steady change in the background levels of the measurements, generally rising as one moves northward or southward from the magnetic equator.Fortunately, it can be removed from the data by a very simple mathematical procedure called, a spherical harmonic expansion, based on magnetic observations made at magnetic observatories around the world. The model that is used for doing this is called the "International Geomagnetic Reference Field" or IGRF, and its value is easily calculated at any given geographical position.From the core of the earth to a depth below the surface where the temperature reaches a value of about 500 degrees C., the "Currie point", there are no additional contributions to the magnetic field. Above the "Currie point", which occurs at depths of between 5 and 15 km., some minerals acquire magnetic properties and, therefore, cause local disturbances, called magnetic anomalies, in the geomagnetic field. These minerals are a small number of oxides of Fe and Ti, and one of the family of pyrrhotites.The most strongly magnetic and the most common magnetic mineral is magnetite. Others include maghemite, the titanomagnetites, and the titanomaghemites. Pyrrhotite is comparatively scarce compared with these ; and so, for practical purposes, magnetic contour maps can be viewed as giving information only on the distribution of magnetic iron oxides (chiefly magnetite) in the rocks that lie above the Currie point.
The intensity of magnetization, I, of a rock or mineral is measured in nano Teslas, nT, and is due chiefly to two factors as shown in equation 1:
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The first term in equation 1 is the "Induced" magnetic field, or the magnetization a rock obtains, by virtue of its susceptibility, through the applied field. It disappears when the rock is removed from the magnetic field.
The intensity of magnetization, however, is not fixed with respect to time and space. In aeromagnetic surveys we are usually interested in the spatial variations of the intensity of magnetization and, thus, the temporal variations must be identified and removed during data compilation. Three main types of temporal variations have been found to cause spurious errors in aeromagnetic data as follows:
| 1. | Diurnal (24 hour variation): |
| This variation usually has an amplitude of about 50 nT to 100 nT and is caused by large scale ionosphereic motions. It is removed by monitoring the field using a base station magnetometer or using a network leveling program. | |
| 2. | Magnetic Storms: |
| These are abrupt variations of several hundred nT and last for several hours. Because they follow cosmic ray activity they are probably related to solar activity. In many cases data collection must stop during a sever storm, hence the importance of using a base station. | |
| 3. | Micropulsations |
| These are very short period, 0.01 sec. to 10 sec. random variations having variable amplitude from about 0.001 nT to 10 nT. There are probably a variety of causes for micropulsations, including atmospheric electromagnetic activity. Micropulsations can be important in high sensitivity surveys. |
An appreciation of the possible severity of the effect of temporal variations, sometimes referred to collectively as "diurnal" variations, can be gained from figure 2.1-1 which shows a comparison between shipboard magnetic variations and base station variation at a high magnetic latitude. Note that some of the "diurnal" fluctuations are as high as 500 nT. Because the ship travels much slower than an aircraft, it collects data over a much longer time period and so longer wavelength temporal variations are more evident in marine data than in airborne data.
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| Figure 2.1-1: A comparison between base station magnetic variations and ship-borne variations |
2.2 Instrumentation
The magnetic field is measured by sampling the total field, or a component of the field, at either discrete times (usually at intervals of from .1 sec. to 1 sec), or by recording the field continuously along the flight lines. While there are at least five different types of magnetometers that are used to collect various types of geophysical data: the Fluxgate, Proton Precession, Optically Pumped, Overhauser, and Squid, (superconducting quantum interference device) only the first three are in common use for aeromagnetic data collection. Thus we will only describe the operation of the three most common types.a) The Fluxgate Magnetometer
A three axis fluxgate magnetometer typicaly is included in all airborne survey systems to support data correction for magnetic interference from the aircraft or other sources. It three axis fluxgate magnetometer records the orientation of the aircraft in the earth's main magnetic field and this information is used to correct the main magnetic measurments for the orientation effects of the aircraft in a process called "magnetic compensation" correction.
b) The Proton Precession Magnetometer
The proton precession magnetometer is based on a property of the atomic nucleus. If the electrons spinning about the nucleus are deflected from the direction of the earth's field the distorted spin axis will precess back to the direction of the earth's field. The precession frequency, called the Larmor frequency, is related to the magnitude of the magnetic field by:
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c) Optically Pumped Magnetometers. (Alkali or He vapor)
The optically pumped magnetometer also depends on the precession property of the atomic nucleus. In this case the sensors bottle contains helium or an alkali vapor. The atoms of the vapor are illuminated by polarized light of a lamp that contains the same vapor as the bottle, and the intensity of the light transmitted through the vapor is recorded by a photo cell. This illumination excites the valence electron of each vapor atom to a higher energy level and the vapor becomes transparent to the light. An external magnetic field also changes the energy level of the atoms. If an alternating magnetic field at the Larmor frequency is applied to the vapor at right angles to the earth's field the atoms energy levels are returned to the lower level where the vapor becomes opaque to the light. A feedback loop between the photo cell and the coil can be established such that the system oscillates at the Larmor frequency and so changes frequency as the intensity of the magnetic field changes.A schematic diagram of a typical optically pumped magnetometer is shown in figure 2.2-1, (Hood, Peter, 1969) |
Figure 2.2-1: Self-oscillating alkali-vapor magnetometer
The instrument will not operate when the ambient magnetic field is either parallel to, or normal to, the optical axis of the gas cell. This requires that the orientation of the sensor be changed, particularly at low latitudes and when crossing the equator.Because of this instruments high accuracy, reasonably wide dynamic range, and ability to sample the field very rapidly, most contractors use this type of magnetometer for all aeromagnetic surveys. The MiniMag magnetometer card included in the Integrated Airborne Geophysical System is designed to operate with up to four optically pumped sensors. The Larmor frequency of the magnetometer is resolved to 1 part in 96,000,000 without filtering, 10 times per second. A special feature of this system is that the aircraft's heading is monitored by the system and is used by the MiniMag magnetometer module to automatically toggle the cesium sensor's polarity as necessary at low magnetic latitude.
During vertical magnetic gradient surveys, two alkali vapor optically pumped magnetometers are operated together at a fixed distance apart and will therefore measure the vertical gradient of the total field, the difference between the two measurements, as well as the total field itself. It is, of course, possible to measure the horizontal gradients as well, but these are not usually geologically useful. Figure 2.2-2 shows a Navajo aircraft with a vertical magnetic gradiometer system installed as a tail stinger. This configuration is ideal for conducting both regional and detailed surveys where the magnetic data is the primary product of the survey and it permits the carrying on other instruments for multi-parameter surveys.
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| Figure 2.2-2: | A airborne survey Navajo aircraft with the twin stinger vertical magnetic gradiometer installed. Each of the two tale stingers houses an optically pumped magnetometer. |
2.3 Survey Operations
Many of the following comments are applicable to all geophysical surveys; not just aeromagnetic surveys. In some cases, the details of the application of the ideas presented here will need to be altered to suit the specific needs of electromagnetic or radiometric data. In the sections where these data types are discussed, we will indicate where the requirements of that specific type of data may differ from those discussed here.The final products of an airborne geophysical survey, in this case aeromagnetic, are usually one or more maps such as the contoured total magnetic field, coloured and/or shadow maps of the total field or a parameter derived from the total field, and a digital data file recorded on a convenient medium that contains the time, location and value of each measurement and any other information relevant to that measurement. Because we actually measure the field only at discrete points in time and along flight lines, all maps, other than profile maps, are an interpolation of the measured data. The selection of survey parameters by the exploration manager, such as line spacing, altitude, and the orientation of the traverse lines as well as compilation and presentation procedures and an evaluation of the anticipated noise - both temporal and geological - should all be made with the desired accuracy of the final products as your guide. These parameters should be detailed in the Contract Specifications. For multi-method surveys, the survey parameters should be chosen to fit the needs of the method most sensitive to them, or in cases of conflict, of the method expected to yield the most useful data. Typical major points that we recommend that should be covered by an airborne survey contract are listed in table 2.3 - 1.When evaluating competitive bids for airborne surveying the following two general catagories should be considered and each proposal graded under these points. 1. Quality of the proposal (Technical), Company Experience,
2. Project personnel (Qualifications), Delivery dates and Price.
Three of the most important factors to be specified for any airborne geophysical survey are:
- The flight height
- the traverse line separation
- The traverse line orientation (direction)
2.3a Survey Design
The selection of traverse line spacing and flight elevation depends critically on the selection of the line direction. The selection of line direction depends on two main consideration, the magnetic inclination in the survey area (sometimes called the magnetic latitude of the area), and the geological strike of the phenomena of most importance to the exploration objective.Figure 2.3-1 shows north-south oriented magnetic profiles across a dipole source at various magnetic latitudes. In the southern hemisphere south is towards the right hand side; in the northern hemisphere north is towards the right. Note that the anomaly produced by the dipole is symmetrical at the pole and at the equator, but is non-symmetrical everywhere else. This implies that the true shape of a magnetic feature is best defined along a north south traverse in most areas of the world. Thus the preferred traverse flight line direction would be north-south if the anomalies in the area were distributed randomly. Because regional surveys are conducted over very large areas usually containing various geological strike directions, a north-south traverse line orientation is usually preferred for aeromagnetic surveys. |
Figure 2.3-1: Profiles of total intensity anomaly from a dipole source at north geomagnetic latitudes where i = 0, 15, 30,45 ,60, 75 and 90 degrees.
However, if the survey area is known to contain a pronounced geological strike direction and the magnetic latitude is either very high or very low it may be advantageous to orient the traverse line direction perpendicular to the geological strike direction. The advantages of this orientation arise because many of the most useful magnetic features arise from linear features like dikes and or faults, and by orienting the traverse lines at right angles to these features, we can be confident that only a few anomalies will not cross the selected flight lines.The respective values of line spacing and height, for all types of geophysical surveys, should be selected in order to reduce the amount of aliasing to less than 5% in the recorded data. Aliasing of data occurs when we try to extract anomalies or signals possessing a wavelength k less than twice the sample, or line, spacing Dx. This idea is illustrated, for simple sine waves, in figure 2.3-2.
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Figure 2.3-2: Illustrating the effect of aliasing on two sine waves having wavelengths of 1/4Dx and 3/4Dx respectively. Note that, when sampled as indicated by the circles, we cannot tell which wave is actually present.
In the limiting case k is called the Nyquist wavelength Kn, where:Kn = 2 Dx
Any anomaly with a "true" wavelength less than Kn will not be identified, and will have the effect of distorting the good data that posses wavelengths longer than the Nyquist.When dealing with an assemblage of magnetic sources it can be shown that the amount of aliasing is simply related to a ratio of the sensor height above the source to the line spacing. In hard rock environments, the sensor height will usually be the distance from the sensor to the surface; however in areas covered by sediments or other non magnetic material, this height will be the flight height plus the thickness of the overlying non-magnetic sediments. As a rule of thumb, the line spacing should equal the sensor height for complete definition of the anomalous magnetic field. However, economic considerations may require a larger line spacing. In this case, the amount of detail required in the survey will depend on the desired use of the data and will, in turn, determine the permissible level of aliasing. Suggested optimum line spacing for given sensor heights is specified in figure 2.3-3 based on a selection of desired products. The larger value in each range may be used if the line direction is perpendicular to the strike of the majority of magnetic structures. |
Figure 2.3-3: Optimum line spacing vs. height for aeromagnetic surveys. Note that line spacings should be smaller if very sophisticated interpretation methods are going to be applied to the data.
Control lines are flown to permit leveling of the survey data. In small surveys, at least three control lines should be flown at right angles to the traverse line direction. In large surveys, control lines should be spaced at intervals of five to ten times the traverse line spacing as is illustrated in figure 2.3-4. |
Figure 2.3-4: A typical flight path pattern flown during geophysical surveys.
Geosteering Drilling System
● Introduction
CGDS-I Near-bit Geosteering Drilling System is configured with CAIMS (China Adjustable Instrument Motor System), WLRS (Wireless Receiver System), CGMWD (China Geosteering MWD) and CFDS (China Formation/Drilling Software System). Besides the steering function, it is capable of measuring engineering and geological parameters while drilling.
● Features
● Measure near-bit geological parameters (Bit Resistivity, Directional Resistivity, Azimuth, Nature Gamma Ray) and near-bit engineering parameters (Inclination, Tool Face) and send the information by wireless transmission technology to the receiving system through steering screw motor
● Near-bit information is integrated into the positive pulser in CGMWD by data connection system, and CGMWD is used as information transmission channel to send the measured downhole information (partial) to surface processing system as the evidence for direction decision
● Downhole motor is used as the steering tool. CFDS processes and analyzes real time near bit geological parameters to interpret and judge nature of fresh footage, then, undertake necessary re-adjustment design, policy making and control while drilling, direct the steering tool into objective oil/gas layer or decide to continue drilling in the oil layer
● Applicable to geological exploration directly and can increase the discovery rate of exploration
● Suitable for the development well in complicated formation or thin oil layer and can increase the probability of penetration and recovery rate
● Reduce drilling costs, increase production and economic profit
● Measurement Flow of CGDS-I System
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