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('70ba138d-0ef3-468a-8e54-03a3be54a366'),
  '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 0x7fc9483d7e90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fc91ec248d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fc91ec29190>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fc91ec29e50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fc91ec2d9d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fc91ec2e610>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fc91ec2ef90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fc91ec33a50>]

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('70ba138d-0ef3-468a-8e54-03a3be54a366'),
  '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 0x7fc9483ef250>,
  <geoh5py.data.float_data.FloatData at 0x7fc9483be290>,
  <geoh5py.data.float_data.FloatData at 0x7fc9483ef1d0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fc91ec24350>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec24e10>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec24990>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7fc91ec247d0>,
  <geoh5py.data.float_data.FloatData at 0x7fc9483ef5d0>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec29350>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fc91ec29250>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec29b10>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec29c50>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7fc91ec29e90>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec2d910>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec2d7d0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fc91ec2d890>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec2e550>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec2e410>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7fc91ec2e5d0>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec2ee50>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec2ef10>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fc91ec33310>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec33b50>,
  <geoh5py.data.float_data.FloatData at 0x7fc91ec33ad0>]}

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).