Magnetotellurics

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.

mtSurvey

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)

/home/docs/checkouts/readthedocs.org/user_builds/mirageoscience-geoh5py/conda/latest/lib/python3.10/site-packages/geoh5py/workspace/workspace.py:1107: UserWarning: From version 0.8.0, the 'h5file' attribute must be a string or path to an existing file, or user must call the 'create' method. We will attempt to `save` the file for you, but this behaviour will be removed in future releases.
  warnings.warn(

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('443178fe-84db-4245-bca3-1d2c47ef417b'),
  '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 0x709a2febabc0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x709a2feb9c00>,
 <geoh5py.groups.property_group.PropertyGroup at 0x709a2febb6d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x709a2febbf40>,
 <geoh5py.groups.property_group.PropertyGroup at 0x709a2febbbe0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x709a2feb9f90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x709a2f594b80>,
 <geoh5py.groups.property_group.PropertyGroup at 0x709a2f5953c0>]

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('443178fe-84db-4245-bca3-1d2c47ef417b'),
  '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 0x709a4003d600>,
  <geoh5py.data.float_data.FloatData at 0x709a4003cd60>,
  <geoh5py.data.float_data.FloatData at 0x709a4003f7f0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x709a4003cd30>,
  <geoh5py.data.float_data.FloatData at 0x709a2feb9f60>,
  <geoh5py.data.float_data.FloatData at 0x709a2feb99c0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x709a2feb9f30>,
  <geoh5py.data.float_data.FloatData at 0x709a2feba9e0>,
  <geoh5py.data.float_data.FloatData at 0x709a2febb5b0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x709a2feb99f0>,
  <geoh5py.data.float_data.FloatData at 0x709a2febb7f0>,
  <geoh5py.data.float_data.FloatData at 0x709a2febbe20>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x709a2febb580>,
  <geoh5py.data.float_data.FloatData at 0x709a2febbdf0>,
  <geoh5py.data.float_data.FloatData at 0x709a2febbac0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x709a2febbd30>,
  <geoh5py.data.float_data.FloatData at 0x709a2febb190>,
  <geoh5py.data.float_data.FloatData at 0x709a2febb040>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x709a2febba90>,
  <geoh5py.data.float_data.FloatData at 0x709a2febaad0>,
  <geoh5py.data.float_data.FloatData at 0x709a2febab60>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x709a2feb98a0>,
  <geoh5py.data.float_data.FloatData at 0x709a2f594ca0>,
  <geoh5py.data.float_data.FloatData at 0x709a2f5952a0>]}

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()
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