Magnetotellurics

Magnetotellurics#

This object can be used to store magnetotelluric (MT) surveys - a natural-source geophysical method. Data are provided in the frequency-domain as point source measurements of either impedances or apparent resistity/phase.

The following example shows how to generate an MT survey with associated data stored in geoh5 format and accessible from Geoscience ANALYST.

mtSurvey

import numpy as np

from geoh5py.objects import MTReceivers
from geoh5py.workspace import Workspace


# Create a new project
workspace = Workspace("my_project.geoh5")

# Define a synthetic survey with receivers on 2 lines, 60 m apart
x_loc, y_loc = np.meshgrid(np.linspace(-5, 5, 2), np.linspace(0.0, 20.0, 9))
vertices = np.c_[x_loc.ravel(), y_loc.ravel(), np.zeros_like(x_loc).ravel()]

# Create the survey from vertices
mt_survey = MTReceivers.create(workspace, vertices=vertices)

/home/docs/checkouts/readthedocs.org/user_builds/mirageoscience-geoh5py/conda/latest/lib/python3.10/site-packages/geoh5py/workspace/workspace.py:1104: UserWarning: From version 0.8.0, the 'h5file' attribute must be a string or path to an existing file, or user must call the 'create' method. We will attempt to `save` the file for you, but this behaviour will be removed in future releases.
  warnings.warn(

Only receivers are needed to define the survey as MT uses the ambient electromagntic field of the Earth - no transmitters (source) required.

Metadata#

Along with the MTReceivers, the metadata contains all the necessary information to define the geophysical experiment.

mt_survey.metadata

{'EM Dataset': {'Channels': [],
  'Input type': 'Rx only',
  'Property groups': [],
  'Receivers': UUID('7bd693d2-548a-492d-9e40-c579d3cfa776'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Channels#

List of frequencies at which the data are provided.

mt_survey.channels = [1.0, 10.0, 100.0]

Input type#

Generic label used in the geoh5 standard for all EM survey entities. Restricted to Rx only in the case of natural sources methods.

Property groups#

List of PropertyGroups defining the various data components (e.g. Zxx (real), Zxy (imag), …). It is not required to supply all components of the impedence tensor, but it is expected that each component contains a list of data channels of length and in the same order as the Channels (one Data per frequency).

The class method add_components_data can help users add data from nested dictionaries. Below is an example using four components:

# Arbitrary data generator using sine functions
def data_fun(c, f):
    return (c + 1.0) * np.sin(f * np.pi * (x_loc * y_loc).ravel() / 200.0)


# Create a nested dictionary of component and frequency data.
data = {
    component: {
        f"{component}_{freq}": {
            "values": (ff + 1) * 1000.0
            + (cc + 1) * 100.0
            + np.arange(vertices.shape[0])
        }
        for ff, freq in enumerate(mt_survey.channels)
    }
    for cc, component in enumerate(
        [
            "Zxx (real)",
            "Zxx (imaginary)",
            "Zxy (real)",
            "Zxy (imaginary)",
            "Zyx (real)",
            "Zyx (imaginary)",
            "Zyy (real)",
            "Zyy (imaginary)",
        ]
    )
}

mt_survey.add_components_data(data)

[<geoh5py.groups.property_group.PropertyGroup at 0x7a0be6baf7c0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a0be6bf7a60>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a0be6bf7340>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a0bdc75bfd0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a0bdc75b0d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a0bdc75a140>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a0bdc375900>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a0bdc376140>]

Metadata are updated immediately to reflect the addition of components:

mt_survey.metadata

{'EM Dataset': {'Channels': [1.0, 10.0, 100.0],
  'Input type': 'Rx only',
  'Property groups': ['Zxx (real)',
   'Zxx (imaginary)',
   'Zxy (real)',
   'Zxy (imaginary)',
   'Zyx (real)',
   'Zyx (imaginary)',
   'Zyy (real)',
   'Zyy (imaginary)'],
  'Receivers': UUID('7bd693d2-548a-492d-9e40-c579d3cfa776'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Data channels associated with each component can be quickly accessed through the BaseEMSurvey.components property:

mt_survey.components

{'Zxx (real)': [<geoh5py.data.float_data.FloatData at 0x7a0bdc75a350>,
  <geoh5py.data.float_data.FloatData at 0x7a0be6bf69e0>,
  <geoh5py.data.float_data.FloatData at 0x7a0be6bf6a70>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7a0be6bf4760>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc75a290>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc75a2c0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7a0bdc75a650>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc75a0b0>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc75a0e0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7a0bdc75b9a0>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc75a260>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc75be80>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7a0bdc3742e0>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc374700>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc374730>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7a0bdc374ac0>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc374f40>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc374f70>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7a0bdc375300>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc3748b0>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc3757e0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7a0bdc375b40>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc3750f0>,
  <geoh5py.data.float_data.FloatData at 0x7a0bdc376020>]}

Receivers#

Generic label used in the geoh5 standard for EM survey to identify the receiver entity. Restricted to itself in the case of MTReceivers.

Survey type#

Label identifier for Magnetotellurics survey type.

Unit#

Units for frequency sampling of the data: Hertz (Hz), KiloHertz (kHz), MegaHertz (MHz) or Gigahertz (GHz).

workspace.close()
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