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/stable/lib/python3.10/site-packages/geoh5py/workspace/workspace.py:1107: 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('df01ca95-9839-4b3e-956c-d3ec9f68e571'),
  '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 0x775f0b3b3610>,
 <geoh5py.groups.property_group.PropertyGroup at 0x775f0b478280>,
 <geoh5py.groups.property_group.PropertyGroup at 0x775f0b4791e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x775f0b479840>,
 <geoh5py.groups.property_group.PropertyGroup at 0x775f0b479660>,
 <geoh5py.groups.property_group.PropertyGroup at 0x775f0b4786d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x775f0b47a650>,
 <geoh5py.groups.property_group.PropertyGroup at 0x775f0b47ae90>]

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('df01ca95-9839-4b3e-956c-d3ec9f68e571'),
  '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 0x775f424b74f0>,
  <geoh5py.data.float_data.FloatData at 0x775f42473370>,
  <geoh5py.data.float_data.FloatData at 0x775f0b3b3a60>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x775f424b7c70>,
  <geoh5py.data.float_data.FloatData at 0x775f424b5ab0>,
  <geoh5py.data.float_data.FloatData at 0x775f0b4783a0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x775f424b79a0>,
  <geoh5py.data.float_data.FloatData at 0x775f0b478160>,
  <geoh5py.data.float_data.FloatData at 0x775f0b4790c0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x775f0b4783d0>,
  <geoh5py.data.float_data.FloatData at 0x775f0b479300>,
  <geoh5py.data.float_data.FloatData at 0x775f0b479930>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x775f0b479090>,
  <geoh5py.data.float_data.FloatData at 0x775f0b479ba0>,
  <geoh5py.data.float_data.FloatData at 0x775f0b479480>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x775f0b479900>,
  <geoh5py.data.float_data.FloatData at 0x775f0b479540>,
  <geoh5py.data.float_data.FloatData at 0x775f0b478820>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x775f0b479450>,
  <geoh5py.data.float_data.FloatData at 0x775f0b478ac0>,
  <geoh5py.data.float_data.FloatData at 0x775f0b47a530>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x775f0b478760>,
  <geoh5py.data.float_data.FloatData at 0x775f0b47a770>,
  <geoh5py.data.float_data.FloatData at 0x775f0b47ad70>]}

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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