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('4506e056-9be9-476a-b9fa-b1a5375a2391'),
  '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):
    (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 0x7f9aac2fc910>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f9aac2db9d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f9aac2db7c0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f9a83607a00>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f9a83605f30>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f9a83607dc0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f9a83606020>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f9a83607cd0>]

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('4506e056-9be9-476a-b9fa-b1a5375a2391'),
  '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 0x7f9a836066e0>,
  <geoh5py.data.float_data.FloatData at 0x7f9aac2fd270>,
  <geoh5py.data.float_data.FloatData at 0x7f9aac2d9120>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f9a83606530>,
  <geoh5py.data.float_data.FloatData at 0x7f9a836069b0>,
  <geoh5py.data.float_data.FloatData at 0x7f9a836069e0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f9a836054b0>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83605e70>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83605ea0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f9a83607430>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83606800>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83607940>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f9a836075e0>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83605d20>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83605ff0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f9a836057e0>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83607910>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83607d00>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f9a83607e50>,
  <geoh5py.data.float_data.FloatData at 0x7f9a83607df0>,
  <geoh5py.data.float_data.FloatData at 0x7f9a836a0550>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f9a836a08e0>,
  <geoh5py.data.float_data.FloatData at 0x7f9a836a0d60>,
  <geoh5py.data.float_data.FloatData at 0x7f9a836a0d30>]}

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()