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('fe48239d-67fb-408d-90a0-524fb66e3325'),
  '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 0x74cd0424e990>,
 <geoh5py.groups.property_group.PropertyGroup at 0x74cd069cd550>,
 <geoh5py.groups.property_group.PropertyGroup at 0x74cccf424e00>,
 <geoh5py.groups.property_group.PropertyGroup at 0x74cccf4256a0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x74cccf425fa0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x74cccf426810>,
 <geoh5py.groups.property_group.PropertyGroup at 0x74cccf427110>,
 <geoh5py.groups.property_group.PropertyGroup at 0x74cccf427a10>]

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('fe48239d-67fb-408d-90a0-524fb66e3325'),
  '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 0x74cccf5cbce0>,
  <geoh5py.data.float_data.FloatData at 0x74cccf77e870>,
  <geoh5py.data.float_data.FloatData at 0x74cccf7d67b0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x74cd21d3b680>,
  <geoh5py.data.float_data.FloatData at 0x74cccf6fc530>,
  <geoh5py.data.float_data.FloatData at 0x74cd06d037a0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x74cccf6a31a0>,
  <geoh5py.data.float_data.FloatData at 0x74cccf5cbe90>,
  <geoh5py.data.float_data.FloatData at 0x74cccf424cb0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x74cccf5cbe00>,
  <geoh5py.data.float_data.FloatData at 0x74cccf425400>,
  <geoh5py.data.float_data.FloatData at 0x74cccf425550>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x74cccf424ce0>,
  <geoh5py.data.float_data.FloatData at 0x74cccf425d00>,
  <geoh5py.data.float_data.FloatData at 0x74cccf425e50>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x74cccf425580>,
  <geoh5py.data.float_data.FloatData at 0x74cccf426600>,
  <geoh5py.data.float_data.FloatData at 0x74cccf426750>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x74cccf425e80>,
  <geoh5py.data.float_data.FloatData at 0x74cccf426f00>,
  <geoh5py.data.float_data.FloatData at 0x74cccf426fc0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x74cccf426780>,
  <geoh5py.data.float_data.FloatData at 0x74cccf427800>,
  <geoh5py.data.float_data.FloatData at 0x74cccf4278c0>]}

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()