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('f47d9f42-2e08-4d95-b341-ca93febe6ba6'),
  '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 0x7a20867f7800>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a2086528bc0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a2086529490>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a2086529d90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a20865298b0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a2086528380>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a208652aa80>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7a208652b380>]

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('f47d9f42-2e08-4d95-b341-ca93febe6ba6'),
  '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 0x7a2087913590>,
  <geoh5py.data.float_data.FloatData at 0x7a20c516e810>,
  <geoh5py.data.float_data.FloatData at 0x7a20c5f9e930>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7a20c431f890>,
  <geoh5py.data.float_data.FloatData at 0x7a20c425fd40>,
  <geoh5py.data.float_data.FloatData at 0x7a2086528a70>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7a20879ad0a0>,
  <geoh5py.data.float_data.FloatData at 0x7a20865291f0>,
  <geoh5py.data.float_data.FloatData at 0x7a2086529340>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7a2086528c50>,
  <geoh5py.data.float_data.FloatData at 0x7a2086529af0>,
  <geoh5py.data.float_data.FloatData at 0x7a2086529c40>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7a20866635f0>,
  <geoh5py.data.float_data.FloatData at 0x7a20c5bfac30>,
  <geoh5py.data.float_data.FloatData at 0x7a20c5a46690>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7a2086529e20>,
  <geoh5py.data.float_data.FloatData at 0x7a2086528c80>,
  <geoh5py.data.float_data.FloatData at 0x7a2086529040>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7a2086529910>,
  <geoh5py.data.float_data.FloatData at 0x7a208652a870>,
  <geoh5py.data.float_data.FloatData at 0x7a208652a930>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7a20865282f0>,
  <geoh5py.data.float_data.FloatData at 0x7a208652b170>,
  <geoh5py.data.float_data.FloatData at 0x7a208652b230>]}

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()