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('47626866-6472-4774-9f63-377293a78f8a'),
  '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 0x73209043b6e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x73209b35f470>,
 <geoh5py.groups.property_group.PropertyGroup at 0x732090499220>,
 <geoh5py.groups.property_group.PropertyGroup at 0x732090499a60>,
 <geoh5py.groups.property_group.PropertyGroup at 0x73209049b0b0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x73209049b920>,
 <geoh5py.groups.property_group.PropertyGroup at 0x732090310260>,
 <geoh5py.groups.property_group.PropertyGroup at 0x732090310b60>]

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('47626866-6472-4774-9f63-377293a78f8a'),
  '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 0x732090def0b0>,
  <geoh5py.data.float_data.FloatData at 0x7320907613d0>,
  <geoh5py.data.float_data.FloatData at 0x732090deffb0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7320904530b0>,
  <geoh5py.data.float_data.FloatData at 0x732090453dd0>,
  <geoh5py.data.float_data.FloatData at 0x7320bd86bcb0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x732090453ce0>,
  <geoh5py.data.float_data.FloatData at 0x73209834ede0>,
  <geoh5py.data.float_data.FloatData at 0x732090499010>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x73209043be30>,
  <geoh5py.data.float_data.FloatData at 0x73209049a540>,
  <geoh5py.data.float_data.FloatData at 0x73209049a690>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7320904992e0>,
  <geoh5py.data.float_data.FloatData at 0x73209049ae10>,
  <geoh5py.data.float_data.FloatData at 0x73209049af60>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x73209049a6c0>,
  <geoh5py.data.float_data.FloatData at 0x73209049b710>,
  <geoh5py.data.float_data.FloatData at 0x73209049b860>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x73209049af90>,
  <geoh5py.data.float_data.FloatData at 0x73209049bf50>,
  <geoh5py.data.float_data.FloatData at 0x73209049bfe0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x73209049b890>,
  <geoh5py.data.float_data.FloatData at 0x732090310950>,
  <geoh5py.data.float_data.FloatData at 0x732090310a10>]}

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()