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.workspace import Workspace
from geoh5py.objects import MTReceivers

# 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., 20., 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('24128453-229d-48a2-b469-1570154cc287'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Channels

List of frequencies at which the data are provided.

mt_survey.channels = [1., 10., 100.]

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
data_fun = lambda c, f: (c+1.) * np.sin(f * np.pi * (x_loc * y_loc).ravel() / 200.)

# Create a nested dictionary of component and frequency data.
data = {
    component : {
        f"{component}_{freq}": {"values": (ff+1)*1000. + (cc+1) * 100. + 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 0x7f8a3497f5b0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8a51f93b50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8a34898850>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8a348990c0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8a34899900>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8a3489a140>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8a3489a9b0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8a3489b1f0>]

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('24128453-229d-48a2-b469-1570154cc287'),
  '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 0x7f8a3497f580>,
  <geoh5py.data.float_data.FloatData at 0x7f8a3497f640>,
  <geoh5py.data.float_data.FloatData at 0x7f8a3497f5e0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f8a51f93790>,
  <geoh5py.data.float_data.FloatData at 0x7f8a51f93af0>,
  <geoh5py.data.float_data.FloatData at 0x7f8a51f92350>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f8a51f936a0>,
  <geoh5py.data.float_data.FloatData at 0x7f8a348981f0>,
  <geoh5py.data.float_data.FloatData at 0x7f8a34898700>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f8a34898220>,
  <geoh5py.data.float_data.FloatData at 0x7f8a348988e0>,
  <geoh5py.data.float_data.FloatData at 0x7f8a34898ac0>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f8a348986d0>,
  <geoh5py.data.float_data.FloatData at 0x7f8a34899360>,
  <geoh5py.data.float_data.FloatData at 0x7f8a34899150>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f8a34898f40>,
  <geoh5py.data.float_data.FloatData at 0x7f8a34899ba0>,
  <geoh5py.data.float_data.FloatData at 0x7f8a34899990>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f8a34899780>,
  <geoh5py.data.float_data.FloatData at 0x7f8a3489a410>,
  <geoh5py.data.float_data.FloatData at 0x7f8a3489a1d0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f8a34899fc0>,
  <geoh5py.data.float_data.FloatData at 0x7f8a3489ac50>,
  <geoh5py.data.float_data.FloatData at 0x7f8a3489aa40>]}

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).