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.

import numpy as np

from geoh5py.objects import MTReceivers
from geoh5py.workspace import Workspace


# Create a new project
workspace = Workspace("my_project.geoh5")

# 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 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('8c1dfb9d-d4fb-4521-a135-32d8bc470baf'),
  '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 PropertyGroups defining the various data components (e.g. Zxx (real), Zxy (imag), …). It is not required to supply all components of the impedence 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 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 0x7f341099e5f0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f3413dd6740>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f3413dd6230>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f3407cce9e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f3407ccf250>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f3407ccfa90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f3407ccf160>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f3407ccf9a0>]

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('8c1dfb9d-d4fb-4521-a135-32d8bc470baf'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Data channels associated with each component can be quickly accessed through the BaseEMSurvey.components property:

mt_survey.components

{'Zxx (real)': [<geoh5py.data.float_data.FloatData at 0x7f3413da60b0>,
  <geoh5py.data.float_data.FloatData at 0x7f3413dd67d0>,
  <geoh5py.data.float_data.FloatData at 0x7f3413dd6590>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f3407cccc10>,
  <geoh5py.data.float_data.FloatData at 0x7f3407ccd030>,
  <geoh5py.data.float_data.FloatData at 0x7f3407ccd060>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f3407ccd1b0>,
  <geoh5py.data.float_data.FloatData at 0x7f3407ccde70>,
  <geoh5py.data.float_data.FloatData at 0x7f3407ccde40>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f3407cce410>,
  <geoh5py.data.float_data.FloatData at 0x7f3407cccd60>,
  <geoh5py.data.float_data.FloatData at 0x7f3407cce920>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f3407ccec80>,
  <geoh5py.data.float_data.FloatData at 0x7f3407cce170>,
  <geoh5py.data.float_data.FloatData at 0x7f3407ccf190>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f3407ccf4f0>,
  <geoh5py.data.float_data.FloatData at 0x7f3407cce8f0>,
  <geoh5py.data.float_data.FloatData at 0x7f3407ccf9d0>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f3407ccfd30>,
  <geoh5py.data.float_data.FloatData at 0x7f3407ccfac0>,
  <geoh5py.data.float_data.FloatData at 0x7f3407d48220>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f3407d485b0>,
  <geoh5py.data.float_data.FloatData at 0x7f3407d48a30>,
  <geoh5py.data.float_data.FloatData at 0x7f3407d48a00>]}

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()