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('e2516d4d-cfca-4372-a91c-d9d884e10902'),
  '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 0x7f4e7c199ea0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f4e84376080>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f4e58756d70>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f4e58757610>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f4e58757e80>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f4e58757520>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f4e58757d90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f4e587d9780>]

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('e2516d4d-cfca-4372-a91c-d9d884e10902'),
  '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 0x7f4e843b6980>,
  <geoh5py.data.float_data.FloatData at 0x7f4e843b61d0>,
  <geoh5py.data.float_data.FloatData at 0x7f4e58756140>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f4e58756440>,
  <geoh5py.data.float_data.FloatData at 0x7f4e58756800>,
  <geoh5py.data.float_data.FloatData at 0x7f4e58756830>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f4e58756170>,
  <geoh5py.data.float_data.FloatData at 0x7f4e587554b0>,
  <geoh5py.data.float_data.FloatData at 0x7f4e58756cb0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f4e58757040>,
  <geoh5py.data.float_data.FloatData at 0x7f4e587551e0>,
  <geoh5py.data.float_data.FloatData at 0x7f4e58757550>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f4e587578b0>,
  <geoh5py.data.float_data.FloatData at 0x7f4e58756c80>,
  <geoh5py.data.float_data.FloatData at 0x7f4e58757dc0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f4e58757e20>,
  <geoh5py.data.float_data.FloatData at 0x7f4e587d8640>,
  <geoh5py.data.float_data.FloatData at 0x7f4e587d85b0>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f4e587d89a0>,
  <geoh5py.data.float_data.FloatData at 0x7f4e587d8e20>,
  <geoh5py.data.float_data.FloatData at 0x7f4e587d8df0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f4e587d91e0>,
  <geoh5py.data.float_data.FloatData at 0x7f4e587d8730>,
  <geoh5py.data.float_data.FloatData at 0x7f4e587d96c0>]}

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()