{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "**Objective:** \n", "\n", "In this tutorial we will create a simple magnetic problem from scratch using the SimPEG framework.\n", "\n", "We are using the integral form of the magnetostatic problem. In the absence of free-currents or changing magnetic field, magnetic material can give rise to a secondary magnetic field according to:\n", "\n", "$$\\vec b = \\frac{\\mu_0}{4\\pi} \\int_{V} \\vec M \\cdot \\nabla \\nabla \\left(\\frac{1}{r}\\right) \\; dV $$\n", "\n", "Where $\\mu_0$ is the magnetic permealitity of free-space, $\\vec M$ is the magnetization per unit volume and $r$ defines the distance between the observed field $\\vec b$ and the magnetized object. Assuming a purely induced response, the strenght of magnetization can be written as:\n", "\n", "$$ \\vec M = \\mu_0 \\kappa \\vec H_0 $$\n", "\n", "where $\\vec H$ is an external inducing magnetic field, and $\\kappa$ the magnetic susceptibility of matter.\n", "As derived by Sharma 1966, the integral can be evaluated for rectangular prisms such that:\n", "\n", "$$ \\vec b(P) = \\mathbf{T} \\cdot \\vec H_0 \\; \\kappa $$\n", "\n", "Where the tensor matrix $\\bf{T}$ relates the three components of magnetization $\\vec M$ to the components of the field $\\vec b$:\n", "\n", "$$\\mathbf{T} =\n", "\t \\begin{pmatrix}\n", " \t\tT_{xx} & T_{xy} & T_{xz} \\\\\n", "\t\tT_{yx} & T_{yy} & T_{yz} \\\\\n", "\t\tT_{zx} & T_{zy} & T_{zz} \n", "\t\\end{pmatrix} $$\n", " \n", "In general, we discretize the earth into a collection of cells, each contributing to the magnetic data such that:\n", "\n", "$$\\vec b(P) = \\sum_{j=1}^{nc} \\mathbf{T}_j \\cdot \\vec H_0 \\; \\kappa_j$$\n", "\n", "giving rise to a linear problem.\n" ] }, { "cell_type": "code", "execution_count": null, "metadata": { "collapsed": true }, "outputs": [], "source": [] }, { "cell_type": "code", "execution_count": 1, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Using matplotlib backend: nbAgg\n", "Populating the interactive namespace from numpy and matplotlib\n" ] }, { "name": "stderr", "output_type": "stream", "text": [ "C:\\Users\\dominiquef.MIRAGEOSCIENCE\\AppData\\Local\\Continuum\\Anaconda\\lib\\site-packages\\IPython\\kernel\\__init__.py:13: ShimWarning: The `IPython.kernel` package has been deprecated. You should import from ipykernel or jupyter_client instead.\n", " \"You should import from ipykernel or jupyter_client instead.\", ShimWarning)\n" ] } ], "source": [ "%matplotlib notebook\n", "%pylab" ] }, { "cell_type": "code", "execution_count": 2, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Efficiency Warning: Interpolation will be slow, use setup.py!\n", "\n", " python setup.py build_ext --inplace\n", " \n" ] } ], "source": [ "from SimPEG import *\n", "import simpegPF as PF" ] }, { "cell_type": "code", "execution_count": 59, "metadata": { "collapsed": false, "scrolled": true }, "outputs": [], "source": [ "# First we need to define the direction of the inducing field\n", "# As a simple case, we pick a vertical inducing field of magnitude 50,000nT. \n", "# From old convention, field orientation is given as an azimuth from North \n", "# (positive clockwise) and dip from the horizontal (positive downward).\n", "H0 = np.array(([90.,0.,50000.]))\n", "\n", "# Assume all induced so the magnetization M is also in the same direction\n", "M = np.array([90,0])\n", "\n", "# Create a mesh\n", "dx = 5.\n", "\n", "hxind = [(dx,5,-1.3), (dx, 20), (dx,5,1.3)]\n", "hyind = [(dx,5,-1.3), (dx, 20), (dx,5,1.3)]\n", "hzind = [(dx,5,-1.3),(5, 10)]\n", "\n", "mesh = Mesh.TensorMesh([hxind, hyind, hzind], 'CCC')\n", "\n", "# Get index of the center\n", "midx = int(mesh.nCx/2)\n", "midy = int(mesh.nCy/2)\n", "\n", "# Assume flat topo for now, so all cells are active\n", "nC = mesh.nC \n", "actv = np.ones(nC)\n", "\n", "\n", "# Create and array of observation points\n", "xr = np.linspace(-20., 20., 20)\n", "yr = np.linspace(-20., 20., 20)\n", "X, Y = np.meshgrid(xr, yr)\n", "Z = np.ones(X.size)*(mesh.vectorNz[-1]+dx) # Let just put the observation flat\n", "\n", "rxLoc = np.c_[Utils.mkvc(X.T), Utils.mkvc(Y.T), Utils.mkvc(Z.T)]\n" ] }, { "cell_type": "code", "execution_count": 60, "metadata": { "collapsed": false }, "outputs": [ { "data": { "text/plain": [ "59.390075000000003" ] }, "execution_count": 60, "metadata": {}, "output_type": "execute_result" } ], "source": [ "Z.max()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Now that we have all our spatial components, we can create our linear system. For a single location and single component of the data, the system would looks like this:\n", "\n", "$$ b_x =\n", "\t\\begin{bmatrix}\n", "\tT_{xx}^1 &... &T_{xx}^{nc} & T_{xy}^1 & ... & T_{xy}^{nc} & T_{xz}^1 & ... & T_{xz}^{nc}\\\\\n", "\t \\end{bmatrix}\n", "\t \\begin{bmatrix}\n", "\t\t\\mathbf{M}_x \\\\ \\mathbf{M}_y \\\\ \\mathbf{M}_z\n", "\t\\end{bmatrix} \\\\ $$\n", "\n", "where each of $T_{xx},\\;T_{xy},\\;T_{xz}$ are [nc x 1] long. For the $y$ and $z$ component, we need the two other rows of the tensor $\\mathbf{T}$.\n", "In our simple induced case, the magnetization direction $\\mathbf{M_x,\\;M_y\\;,Mz}$ are known and assumed to be constant everywhere, so we can reduce the size of the system such that: \n", "\n", "$$ \\vec{\\mathbf{d}}_{\\text{pred}} = (\\mathbf{T\\cdot M})\\; \\kappa$$\n", "\n", "\n", "\n", "In most geophysical surveys, we are not collecting all three components, but rather the magnitude of the field, or $Total\\;Magnetic\\;Intensity$ (TMI) data.\n", "Because the inducing field is really large, we will assume that the anomalous fields are parallel to $H_0$:\n", "\n", "$$ d^{TMI} = \\hat H_0 \\cdot \\vec d$$\n", "\n", "We then end up with a much smaller system:\n", "\n", "$$ d^{TMI} = \\mathbf{F\\; \\kappa}$$\n", "\n", "where $\\mathbf{F} \\in \\mathbb{R}^{nd \\times nc}$ is our $forward$ operator." ] }, { "cell_type": "code", "execution_count": 61, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Begin calculation of forward operator: tmi\n", "Done 0.0 %\n", "Done 10.0 %\n", "Done 20.0 %\n", "Done 30.0 %\n", "Done 40.0 %\n", "Done 50.0 %\n", "Done 60.0 %\n", "Done 70.0 %\n", "Done 80.0 %\n", "Done 90.0 %\n", "Done 100% ...forward operator completed!!\n", "\n" ] } ], "source": [ "# First, convert the magnetization direction to Cartesian\n", "mi = np.ones(mesh.nC) * M[0]\n", "md = np.ones(mesh.nC) * M[1]\n", "M_xyz = PF.Magnetics.dipazm_2_xyz( mi , md ) # Ouputs an nc x 3 array\n", "\n", "# Create the forward model operator\n", "F = PF.Magnetics.Intrgl_Fwr_Op(mesh,H0,M_xyz,rxLoc,actv,'tmi')" ] }, { "cell_type": "code", "execution_count": 91, "metadata": { "collapsed": false }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Begin calculation of distance weighting for R= 3.0\n", "Done 0.0 %\n", "Done 10.0 %\n", "Done 20.0 %\n", "Done 30.0 %\n", "Done 40.0 %\n", "Done 50.0 %\n", "Done 60.0 %\n", "Done 70.0 %\n", "Done 80.0 %\n", "Done 90.0 %\n", "Done 100% ...distance weighting completed!!\n", "\n" ] }, { "data": { "application/javascript": [ "/* Put everything inside the global mpl namespace */\n", "window.mpl = {};\n", "\n", "mpl.get_websocket_type = function() {\n", " if (typeof(WebSocket) !== 'undefined') {\n", " return WebSocket;\n", " } else if (typeof(MozWebSocket) !== 'undefined') {\n", " return MozWebSocket;\n", " } else {\n", " alert('Your browser does not have WebSocket support.' +\n", " 'Please try Chrome, Safari or Firefox ≥ 6. 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