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.

from pathlib import Path

import numpy as np

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


# Create a new project
h5file = Path("my_project.geoh5")
workspace = Workspace(h5file) if h5file.exists() else Workspace.create(h5file)

# 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)

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 geoh5py.objects.surveys.electromagnetics.magnetotellurics.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('6e02d251-ab9b-4de4-aba5-24d43b11902c'),
  '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 module-geoh5py.groups.property_group.PropertyGroups defining the various data components (e.g. Zxx (real), Zxy (imag), …). It is not required to supply all components of the impedance 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 geoh5py.objects.surveys.electromagnetics.base.BaseEMSurvey.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 0x7d5cc5776f60>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d5cc3bfc2c0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d5cc3bfdcd0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d5cc3bfe5a0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d5cc3bfeea0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d5cc3bff6e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d5cc3bffd10>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d5cc3bfcaa0>]

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('6e02d251-ab9b-4de4-aba5-24d43b11902c'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Data channels associated with each component can be quickly accessed through the geoh5py.objects.surveys.electromagnetics.base.BaseEMSurvey.components property:

mt_survey.components

{'Zxx (real)': [<geoh5py.data.float_data.FloatData at 0x7d5cc4ac2a80>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3b3e150>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3dafa10>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7d5d14b314c0>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3cfbef0>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bfc230>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7d5cc3c6a270>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bfda00>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bfdb50>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7d5cc3bfc380>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bfe330>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc57de690>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7d5cc3bfdb80>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bfec00>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bfed50>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7d5cc3bfe480>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc5776ff0>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bff620>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7d5cc3bfed80>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bffdd0>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bfff20>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7d5cc3bff650>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3bff7d0>,
  <geoh5py.data.float_data.FloatData at 0x7d5cc3a3c7a0>]}

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()