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.

mtSurvey

[1]:

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.

[2]:

mt_survey.metadata

[2]:

{'EM Dataset': {'Channels': [],
  'Input type': 'Rx only',
  'Property groups': [],
  'Receivers': UUID('5b423786-4c5e-4b14-904d-3b7c47bd1960'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Channels¶

List of frequencies at which the data are provided.

[3]:

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:

[4]:

# 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)

[4]:

[<geoh5py.groups.property_group.PropertyGroup at 0x7f7285489420>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f72b0451fc0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f728538c850>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f728538d0c0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f728538d900>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f728538e140>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f728538e9b0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f728538f1f0>]

Metadata are updated immediately to reflect the addition of components:

[5]:

mt_survey.metadata

[5]:

{'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('5b423786-4c5e-4b14-904d-3b7c47bd1960'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Data channels associated with each component can be quickly accessed through the BaseEMSurvey.components property:

[6]:

mt_survey.components

[6]:

{'Zxx (real)': [<geoh5py.data.float_data.FloatData at 0x7f7285488ca0>,
  <geoh5py.data.float_data.FloatData at 0x7f7285489390>,
  <geoh5py.data.float_data.FloatData at 0x7f7285488b80>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f72b041f4c0>,
  <geoh5py.data.float_data.FloatData at 0x7f728538c370>,
  <geoh5py.data.float_data.FloatData at 0x7f728538c310>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f72b04506d0>,
  <geoh5py.data.float_data.FloatData at 0x7f728538c760>,
  <geoh5py.data.float_data.FloatData at 0x7f728538c4f0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f728538c0d0>,
  <geoh5py.data.float_data.FloatData at 0x7f728538ca30>,
  <geoh5py.data.float_data.FloatData at 0x7f728538d0f0>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f728538cfa0>,
  <geoh5py.data.float_data.FloatData at 0x7f728538d330>,
  <geoh5py.data.float_data.FloatData at 0x7f728538d930>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f728538d7e0>,
  <geoh5py.data.float_data.FloatData at 0x7f728538db70>,
  <geoh5py.data.float_data.FloatData at 0x7f728538e170>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f728538e020>,
  <geoh5py.data.float_data.FloatData at 0x7f728538e3e0>,
  <geoh5py.data.float_data.FloatData at 0x7f728538e9e0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f728538e890>,
  <geoh5py.data.float_data.FloatData at 0x7f728538ec20>,
  <geoh5py.data.float_data.FloatData at 0x7f728538f220>]}

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).