# Read and Visualise Geophysical Potential-Fields

Geophysical potential fields (gravity and magnetics) can be calculated directly from the generated kinematic model. A wide range of options also exists to consider effects of geological events on the relevant rock properties. We will here use pynoddy to simply and quickly test the effect of changing geological structures on the calculated geophysical response.

    %matplotlib inline

    import sys, os
    import matplotlib.pyplot as plt
    # adjust some settings for matplotlib
    from matplotlib import rcParams
    # print rcParams
    rcParams['font.size'] = 15
    # determine path of repository to set paths corretly below
    repo_path = os.path.realpath('../..')
    import pynoddy

    import matplotlib.pyplot as plt
    import numpy as np

    from IPython.core.display import HTML
    css_file = 'pynoddy.css'
    HTML(open(css_file, "r").read())

## Read history file from Virtual Explorer

Many Noddy models are available on the site of the Virtual Explorer in the Structural Geophysics Atlas. We will download and use one of these models here as the base model.

We start with the history file of a “Fold and Thrust Belt” setting stored on:

`http://tectonique.net/asg/ch3/ch3_5/his/fold_thrust.his`

The file can directly be downloaded and opened with pynoddy:

    import pynoddy.history
    reload(pynoddy.history)

    his = pynoddy.history.NoddyHistory(url = \
                "http://tectonique.net/asg/ch3/ch3_5/his/fold_thrust.his")

    his.determine_model_stratigraphy()

    his.change_cube_size(50)

    # Save to (local) file to compute and visualise model
    history_name = "fold_thrust.his"
    his.write_history(history_name)
    # his = pynoddy.history.NoddyHistory(history_name)

    output = "fold_thrust_out"
    pynoddy.compute_model(history_name, output)

    ''

    import pynoddy.output
    # load and visualise model
    h_out = pynoddy.output.NoddyOutput(output)

    # his.determine_model_stratigraphy()
    h_out.plot_section('x',
                       layer_labels = his.model_stratigraphy,
                       colorbar_orientation = 'horizontal',
                       colorbar=False,
                       title = '',
    #                   savefig=True, fig_filename = 'fold_thrust_NS_section.eps',
                       cmap = 'YlOrRd')

png

    h_out.plot_section('y', layer_labels = his.model_stratigraphy,
                       colorbar_orientation = 'horizontal', title = '', cmap = 'YlOrRd',
    #                   savefig=True, fig_filename = 'fold_thrust_EW_section.eps',
                       ve=1.5)

png

    h_out.export_to_vtk(vtk_filename = "fold_thrust")

## Visualise calculated geophysical fields

The first step is to recompute the model with the generation of the geophysical responses

    pynoddy.compute_model(history_name, output, sim_type = 'GEOPHYSICS')

    ''

We now get two files for the caluclated fields: ‘.grv’ for gravity, and ‘.mag’ for the magnetic field. We can extract the information of these files for visualisation and further processing in python:

    reload(pynoddy.output)
    geophys = pynoddy.output.NoddyGeophysics(output)

    fig = plt.figure(figsize = (5,5))
    ax = fig.add_subplot(111)
    # imshow(geophys.grv_data, cmap = 'jet')
    # define contour levels
    levels = np.arange(322,344,1)
    cf = ax.contourf(geophys.grv_data, levels, cmap = 'gray', vmin = 324, vmax = 342)
    cbar = plt.colorbar(cf, orientation = 'horizontal')
    # print levels

png

## Change history and compare gravity

As a next step, we will now change aspects of the geological history (paramtereised in as parameters of the kinematic events) and calculate the effect on the gravity. Then, we will compare the changed gravity field to the original field.

Let’s have a look at the properties of the defined faults in the original model:

    for i in range(4):
        print("\nEvent %d" % (i+2))
        print "Event type:\t" + his.events[i+2].event_type
        print "Fault slip:\t%.1f" % his.events[i+2].properties['Slip']
        print "Fault dip:\t%.1f" % his.events[i+2].properties['Dip']
        print "Dip direction:\t%.1f" % his.events[i+2].properties['Dip Direction']

    Event 2
    Event type: FAULT
    Fault slip: -5000.0
    Fault dip:  0.0
    Dip direction:  90.0

    Event 3
    Event type: FAULT
    Fault slip: -3000.0
    Fault dip:  0.0
    Dip direction:  90.0

    Event 4
    Event type: FAULT
    Fault slip: -3000.0
    Fault dip:  0.0
    Dip direction:  90.0

    Event 5
    Event type: FAULT
    Fault slip: 12000.0
    Fault dip:  80.0
    Dip direction:  170.0

    reload(pynoddy.history)
    reload(pynoddy.events)
    his2 = pynoddy.history.NoddyHistory("fold_thrust.his")

    print his2.events[6].properties

    {'Dip': 130.0, 'Cylindricity': 0.0, 'Wavelength': 12000.0, 'Amplitude': 1000.0, 'Pitch': 0.0, 'Y': 0.0, 'X': 0.0, 'Single Fold': 'FALSE', 'Z': 0.0, 'Type': 'Fourier', 'Dip Direction': 110.0}

As a simple test, we are changing the fault slip for all the faults and simply add 1000 m to all defined slips. In order to not mess up the original model, we are creating a copy of the history object first:

    import copy
    his = pynoddy.history.NoddyHistory(history_name)
    his.all_events_end += 1
    his_changed = copy.deepcopy(his)

    # change parameters of kinematic events
    slip_change = 2000.
    wavelength_change = 2000.
    # his_changed.events[3].properties['Slip'] += slip_change
    # his_changed.events[5].properties['Slip'] += slip_change
    # change fold wavelength
    his_changed.events[6].properties['Wavelength'] += wavelength_change
    his_changed.events[6].properties['X'] += wavelength_change/2.

We now write the adjusted history back to a new history file and then calculate the updated gravity field:

    his_changed.write_history('fold_thrust_changed.his')

    # %%timeit
    # recompute block model
    pynoddy.compute_model('fold_thrust_changed.his', 'fold_thrust_changed_out')

    ''

    # %%timeit
    # recompute geophysical response
    pynoddy.compute_model('fold_thrust_changed.his', 'fold_thrust_changed_out',
                          sim_type = 'GEOPHYSICS')

    ''

    # load changed block model
    geo_changed = pynoddy.output.NoddyOutput('fold_thrust_changed_out')
    # load output and visualise geophysical field
    geophys_changed = pynoddy.output.NoddyGeophysics('fold_thrust_changed_out')

    fig = plt.figure(figsize = (5,5))
    ax = fig.add_subplot(111)
    # imshow(geophys_changed.grv_data, cmap = 'jet')
    cf = ax.contourf(geophys_changed.grv_data, levels, cmap = 'gray', vmin = 324, vmax = 342)
    cbar = plt.colorbar(cf, orientation = 'horizontal')

png

    fig = plt.figure(figsize = (5,5))
    ax = fig.add_subplot(111)
    # imshow(geophys.grv_data - geophys_changed.grv_data, cmap = 'jet')
    maxval = np.ceil(np.max(np.abs(geophys.grv_data - geophys_changed.grv_data)))
    # comp_levels = np.arange(-maxval,1.01 * maxval, 0.05 * maxval)
    cf = ax.contourf(geophys.grv_data - geophys_changed.grv_data, 20,
                     cmap = 'spectral')
    cbar = plt.colorbar(cf, orientation = 'horizontal')

png

    # compare sections through model
    geo_changed.plot_section('y', colorbar = False)
    h_out.plot_section('y', colorbar = False)

png

png

    for i in range(4):
        print("Event %d" % (i+2))
        print his.events[i+2].properties['Slip']
        print his.events[i+2].properties['Dip']
        print his.events[i+2].properties['Dip Direction']

    Event 2
    -5000.0
    0.0
    90.0
    Event 3
    -3000.0
    0.0
    90.0
    Event 4
    -3000.0
    0.0
    90.0
    Event 5
    12000.0
    80.0
    170.0

    # recompute the geology blocks for comparison:
    pynoddy.compute_model('fold_thrust_changed.his', 'fold_thrust_changed_out')

    ''

    geology_changed = pynoddy.output.NoddyOutput('fold_thrust_changed_out')

    geology_changed.plot_section('x',
    #                    layer_labels = his.model_stratigraphy,
                       colorbar_orientation = 'horizontal',
                       colorbar=False,
                       title = '',
    #                   savefig=True, fig_filename = 'fold_thrust_NS_section.eps',
                       cmap = 'YlOrRd')

png

    geology_changed.plot_section('y',
                                 # layer_labels = his.model_stratigraphy,
                       colorbar_orientation = 'horizontal', title = '', cmap = 'YlOrRd',
    #                   savefig=True, fig_filename = 'fold_thrust_EW_section.eps',
                       ve=1.5)

png

    # Calculate block difference and export as VTK for 3-D visualisation:
    import copy
    diff_model = copy.deepcopy(geology_changed)
    diff_model.block -= h_out.block

    diff_model.export_to_vtk(vtk_filename = "diff_model_fold_thrust_belt")

## Figure with all results

We now create a figure with the gravity field of the original and the changed model, as well as a difference plot to highlight areas with significant changes. This example also shows how additional equations can easily be combined with pynoddy classes.

    fig = plt.figure(figsize=(20,8))
    ax1 = fig.add_subplot(131)
    # original plot
    levels = np.arange(322,344,1)
    cf1 = ax1.contourf(geophys.grv_data, levels, cmap = 'gray', vmin = 324, vmax = 342)
    # cbar1 = ax1.colorbar(cf1, orientation = 'horizontal')
    fig.colorbar(cf1, orientation='horizontal')
    ax1.set_title('Gravity of original model')

    ax2 = fig.add_subplot(132)

    cf2 = ax2.contourf(geophys_changed.grv_data, levels, cmap = 'gray', vmin = 324, vmax = 342)
    ax2.set_title('Gravity of changed model')
    fig.colorbar(cf2, orientation='horizontal')

    ax3 = fig.add_subplot(133)

    comp_levels = np.arange(-10.,10.1,0.25)
    cf3 = ax3.contourf(geophys.grv_data - geophys_changed.grv_data, comp_levels, cmap = 'RdBu_r')
    ax3.set_title('Gravity difference')

    fig.colorbar(cf3, orientation='horizontal')

    plt.savefig("grav_ori_changed_compared.eps")

png