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('c826c509-3100-4ef2-a4ca-21e7d0e3be16'),
  '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 0x7f0790352290>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0790352740>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f07903220e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f07670c73d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f07670c7c40>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f07670c72e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f07670c7b50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0766d01540>]

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('c826c509-3100-4ef2-a4ca-21e7d0e3be16'),
  '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 0x7f07670c5ed0>,
  <geoh5py.data.float_data.FloatData at 0x7f0790352710>,
  <geoh5py.data.float_data.FloatData at 0x7f0790322020>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f07670c6110>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c4cd0>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c4d30>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f07670c5fc0>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c6a10>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c69e0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f07670c6e00>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c4f40>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c7310>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f07670c7670>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c6b60>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c7b80>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f07670c7ee0>,
  <geoh5py.data.float_data.FloatData at 0x7f07670c7c70>,
  <geoh5py.data.float_data.FloatData at 0x7f0766d003d0>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f0766d00760>,
  <geoh5py.data.float_data.FloatData at 0x7f0766d00be0>,
  <geoh5py.data.float_data.FloatData at 0x7f0766d00bb0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f0766d00fa0>,
  <geoh5py.data.float_data.FloatData at 0x7f0766d00490>,
  <geoh5py.data.float_data.FloatData at 0x7f0766d01480>]}

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()