Environmental and Exploration Geophysics I

Environmental and Exploration Geophysics I Magnetic Methods (II) tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Tom Wilson, Department of Geology and Geography

Magnetic field variations are generally of non-geologic origin Long term drift in magnetic declination and inclination Tom Wilson, Department of Geology and Geography

Magnetic Field Variations – annual drift of the magnetic pole Tom Wilson, Department of Geology and Geography

Diurnal variations in the Earth’s Magnetic field Tom Wilson, Department of Geology and Geography

Magnetic fields like gravitational fields are not constant. However, magnetic field variations are much more erratic and unpredictable Diurnal variations http://www.earthsci.unimelb.edu.au/ES304 /MODULES/ MAG/NOTES/tempcorrect.html Tom Wilson, Department of Geology and Geography

Solar activity and sunspot cycles Nov. 19th 2009 Tom Wilson, Department of Geology and Geography

Micropulsations Today’s Space Weather http://www.swpc.noaa.gov/today.html Real Time Magnetic field data http://www.swpc.noaa.gov/ace/ace_rtsw_data.html Tom Wilson, Department of Geology and Geography

http://www.swpc.noaa.gov/ace/ace_rtsw_data.html From the Advanced Composition Explorer Satellite Tom Wilson, Department of Geology and Geography

Field Between Reversals http://www.es.ucsc.edu/~glatz/geodynamo.html Normal dipolar field Tom Wilson, Department of Geology and Geography

Corrections? In general there are few corrections to apply to magnetic data. The largest non-geological variations in the earth’s magnetic field are those associated with diurnal variations, micropulsations and magnetic storms. The vertical gradient of the vertical component of the earth’s magnetic field at this latitude is approximately 0.025nT/m. This translates into 1nT per 40 meters. The magnetometer we have been using in the field reads to a sensitivity of 1nT and the anomalies we observed at the Falls Run site are of the order of 200 nT or more. Hence, elevation corrections are generally not needed. Variations of total field intensity as a function of latitude are also relatively small (0.00578nT/m). The effect at Falls Run would have been about 1/2 nT from north to south across the site. International geomagnetic reference formula Tom Wilson, Department of Geology and Geography

Correcting for Diurnal Variations Reoccupy the base The single most important correction to make is one that compensates for diurnal variations, micropulsations and magnetic storms. This is usually done by reoccupying a base station periodically throughout the duration of a survey to determine how total field intensity varies with time and to eliminate these variations in much the same way that tidal and instrument drift effects were eliminated from gravity observations. Tom Wilson, Department of Geology and Geography

Anomalies - Total Field and Residual The regional field can be removed by surface fitting and line fitting procedures identical to those used in the analysis of gravity data. Tom Wilson, Department of Geology and Geography

Magnetic susceptibility is a key parameter, however, it is so highly variable for any given lithology that estimates of k obtained through inverse modeling do not necessarily indicate that an anomaly is due to any one specific rock type. Tom Wilson, Department of Geology and Geography

+ - + - S N The induced magnetic field of a metallic drum N F E S The Earth’s main field Tom Wilson, Department of Geology and Geography

Vector Awareness N S Tom Wilson, Department of Geology and Geography

Cross sectional area A + n turns l - Magnetic fields are fundamentally associated with circulating electric currents; thus we can also formalize concepts like pole strength, dipole moment, etc. in terms of current flow relationships. pl = n iA pl is the dipole moment Tom Wilson, Department of Geology and Geography

I=kF Hysterisis Loops I is the intensity of magnetization and FE is the ambient (for example - Earth’s) magnetic field intensity. k is the magnetic susceptibility. Tom Wilson, Department of Geology and Geography

The intensity of magnetization is equivalent to the magnetic moment per unit volume or Magnetic dipole moment per unit volume where and also, . Thus yielding and The cgs unit for pole strength is the ups Tom Wilson, Department of Geology and Geography

Recall from our earlier discussions that magnetic field intensity so that Thus providing additional relationships that may prove useful in problem solving exercises. For example, Tom Wilson, Department of Geology and Geography

What does this tell us about units of these different quantities? Summary We refer to the magnetic field intensity as H (or as Burger et al. do, F) Tom Wilson, Department of Geology and Geography

Basic Magnetic Units and Vector Concepts Problem 2 Observation point What is the resultant? At a point 20 cm from the center of a thin magnetized rod 40 cm long and equidistant from its ends, the magnetic field is 500 nT. What is the pole strength in Oersteds? Tom Wilson, Department of Geology and Geography

x and y components of field associated with each pole Tom Wilson, Department of Geology and Geography

Sum x and y components to get the resultant field Tom Wilson, Department of Geology and Geography

Problem - At a point 20 cm from the center of a thin magnetized rod 40 cm long and equidistant from its ends, the magnetic field is 500 nT. What is the pole strength in Oersted-cm2? Tom Wilson, Department of Geology and Geography

Sign conventions assume that the test pole is positive. Tom Wilson, Department of Geology and Geography

Resultant x and y components HR=2Hx=500nT Tom Wilson, Department of Geology and Geography

The different ways of expressing magnetic field intensity lead to different units; ups/cm2, Oersteds & nanoTeslas Tom Wilson, Department of Geology and Geography

Some units interrelationships 105 Tom Wilson, Department of Geology and Geography

HRX = 500nT Tom Wilson, Department of Geology and Geography

Then, what is H+ or H-? Once we know this, we can then determine the pole strength. H = p/r2 so p = Hr2 Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Where we started ... Since the bedrock is magnetic, we have no way of differentiating between anomalies produced by bedrock and those ? produced by buried storage drums. Tom Wilson, Department of Geology and Geography

Why gravity? Acquisition of gravity data allows us to estimate variations in bedrock depth across the profile. With this knowledge, we can directly calculate the contribution of bedrock to the magnetic field observed across the profile. Tom Wilson, Department of Geology and Geography

How many drums are represented by the triangular-shaped object you entered into your model? Use the magic eye to get the coordinates of the polygon defining the drums Plot the corner coordinates for the triangular shaped object you derived at 1:1 scale and compute the area. Tom Wilson, Department of Geology and Geography

How many drums? Area of one drum ~ 4 square feet What’s wrong with the format of this plot? We’ll talk more about the last bullet (1/r3) on the results-to-be-discussed list a little later. Tom Wilson, Department of Geology and Geography

Anomaly associated with buried metallic materials Computed magnetic field produced by bedrock Results obtained from inverse modeling Bedrock configuration determined from gravity survey Introduction to the magnetics computer lab Tom Wilson, Department of Geology and Geography

Let’s continue with the second part of the magnetics lab Where are the drums and how many are there? Tom Wilson, Department of Geology and Geography

After Thanksgiving Break ... Bring questions to class Dec 1st Problems 1 & 2 will be due on Thursday, December 3rd Magnetics Lab is due on Tuesday, Dec 8th Magnetic paper summaries are due Thursday December 10th Exam, Thursday December 17th; 3-5pm Questions? Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics I