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('ce67ef06-d8bc-4581-aaca-ad5e00772da1'),
  '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 0x7996a01abd40>,
 <geoh5py.groups.property_group.PropertyGroup at 0x799652188c50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7996521e4800>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7996521e5100>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7996521e5a00>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7996521e6240>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7996521e6b40>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7996521e7440>]

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('ce67ef06-d8bc-4581-aaca-ad5e00772da1'),
  '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 0x7996780ca6c0>,
  <geoh5py.data.float_data.FloatData at 0x799653476ae0>,
  <geoh5py.data.float_data.FloatData at 0x799652117f80>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7996a001d940>,
  <geoh5py.data.float_data.FloatData at 0x7996530f6b10>,
  <geoh5py.data.float_data.FloatData at 0x79965218b200>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x799652253ef0>,
  <geoh5py.data.float_data.FloatData at 0x79965218bb00>,
  <geoh5py.data.float_data.FloatData at 0x7996521e4680>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x79965218b1d0>,
  <geoh5py.data.float_data.FloatData at 0x7996521e4e60>,
  <geoh5py.data.float_data.FloatData at 0x7996521e4fb0>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7996521e46b0>,
  <geoh5py.data.float_data.FloatData at 0x7996521e5760>,
  <geoh5py.data.float_data.FloatData at 0x7996521e58b0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7996521e4fe0>,
  <geoh5py.data.float_data.FloatData at 0x7996521e6030>,
  <geoh5py.data.float_data.FloatData at 0x7996521e6180>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7996521e58e0>,
  <geoh5py.data.float_data.FloatData at 0x7996521e6930>,
  <geoh5py.data.float_data.FloatData at 0x7996521e69f0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7996521e61b0>,
  <geoh5py.data.float_data.FloatData at 0x7996521e7230>,
  <geoh5py.data.float_data.FloatData at 0x7996521e72f0>]}

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()