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('0d97d47b-d2c2-4c9e-9a80-75d58d258e3f'),
  '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 0x70f0e5246630>,
 <geoh5py.groups.property_group.PropertyGroup at 0x70f135f88fb0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x70f0e5295fa0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x70f0e5297410>,
 <geoh5py.groups.property_group.PropertyGroup at 0x70f0e5297d10>,
 <geoh5py.groups.property_group.PropertyGroup at 0x70f0e510c590>,
 <geoh5py.groups.property_group.PropertyGroup at 0x70f0e510ce90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x70f0e510d790>]

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('0d97d47b-d2c2-4c9e-9a80-75d58d258e3f'),
  '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 0x70f13413bc20>,
  <geoh5py.data.float_data.FloatData at 0x70f0e645fb60>,
  <geoh5py.data.float_data.FloatData at 0x70f1342377d0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x70f135e1d850>,
  <geoh5py.data.float_data.FloatData at 0x70f12c1ed760>,
  <geoh5py.data.float_data.FloatData at 0x70f12c2d2d80>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x70f12c11eff0>,
  <geoh5py.data.float_data.FloatData at 0x70f135ee9250>,
  <geoh5py.data.float_data.FloatData at 0x70f134288e00>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x70f135ee91f0>,
  <geoh5py.data.float_data.FloatData at 0x70f0e5297110>,
  <geoh5py.data.float_data.FloatData at 0x70f0e52972c0>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x70f0e5296510>,
  <geoh5py.data.float_data.FloatData at 0x70f0e5297a70>,
  <geoh5py.data.float_data.FloatData at 0x70f0e5297bc0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x70f0e52972f0>,
  <geoh5py.data.float_data.FloatData at 0x70f12c01ffe0>,
  <geoh5py.data.float_data.FloatData at 0x70f0e510c4d0>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x70f0e5297bf0>,
  <geoh5py.data.float_data.FloatData at 0x70f0e510cc80>,
  <geoh5py.data.float_data.FloatData at 0x70f0e510cd40>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x70f0e510c500>,
  <geoh5py.data.float_data.FloatData at 0x70f0e510d580>,
  <geoh5py.data.float_data.FloatData at 0x70f0e510d640>]}

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()