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('b86d0909-a1c9-49c8-aa0c-5e7827b8cedc'),
  '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 0x7d345c29e360>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d3424628950>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d34247db410>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d342462a540>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d34247db740>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d345e5db200>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d3424664740>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7d342462bfe0>]

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('b86d0909-a1c9-49c8-aa0c-5e7827b8cedc'),
  '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 0x7d344593b740>,
  <geoh5py.data.float_data.FloatData at 0x7d342486b9e0>,
  <geoh5py.data.float_data.FloatData at 0x7d345c3eab40>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7d34249053d0>,
  <geoh5py.data.float_data.FloatData at 0x7d34256593d0>,
  <geoh5py.data.float_data.FloatData at 0x7d3424628650>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7d345ddd7500>,
  <geoh5py.data.float_data.FloatData at 0x7d34246289e0>,
  <geoh5py.data.float_data.FloatData at 0x7d3424629310>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7d34247db890>,
  <geoh5py.data.float_data.FloatData at 0x7d3424629dc0>,
  <geoh5py.data.float_data.FloatData at 0x7d342462a180>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7d34247dba10>,
  <geoh5py.data.float_data.FloatData at 0x7d342462acf0>,
  <geoh5py.data.float_data.FloatData at 0x7d342462b020>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7d342462a690>,
  <geoh5py.data.float_data.FloatData at 0x7d34247dbd40>,
  <geoh5py.data.float_data.FloatData at 0x7d34458f5130>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7d34247dbd70>,
  <geoh5py.data.float_data.FloatData at 0x7d342462ac00>,
  <geoh5py.data.float_data.FloatData at 0x7d34246296a0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7d34247db9e0>,
  <geoh5py.data.float_data.FloatData at 0x7d3424629220>,
  <geoh5py.data.float_data.FloatData at 0x7d345c29db20>]}

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()