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('861b7db4-d2e1-4848-b832-2b11d00701b4'),
  '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 0x7f47fc115ed0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f47d8074340>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f48002a9570>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f47d8074fa0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f47d80757e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f47d8076020>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f47d8076890>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f47d80770d0>]

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('861b7db4-d2e1-4848-b832-2b11d00701b4'),
  '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 0x7f47d816d150>,
  <geoh5py.data.float_data.FloatData at 0x7f47fc1143d0>,
  <geoh5py.data.float_data.FloatData at 0x7f47fc115ea0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f47fc114dc0>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8074250>,
  <geoh5py.data.float_data.FloatData at 0x7f47d80741f0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f48002a9cf0>,
  <geoh5py.data.float_data.FloatData at 0x7f48002a93f0>,
  <geoh5py.data.float_data.FloatData at 0x7f48002a9ab0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f47d80740a0>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8074160>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8074fd0>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f47d8074e80>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8075210>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8075810>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f47d80756c0>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8075a50>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8076050>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f47d8075f00>,
  <geoh5py.data.float_data.FloatData at 0x7f47d80762c0>,
  <geoh5py.data.float_data.FloatData at 0x7f47d80768c0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f47d8076770>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8076b00>,
  <geoh5py.data.float_data.FloatData at 0x7f47d8077100>]}

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).