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('a73a199b-c5e0-469c-9b8a-f456e7a05453'),
  '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 0x7feaac13b3a0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fea80eb98a0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fea80eba170>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fea80ebb160>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fea80ebb9a0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fea80d40220>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fea80d40a90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fea80d412d0>]

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('a73a199b-c5e0-469c-9b8a-f456e7a05453'),
  '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 0x7feaac13a890>,
  <geoh5py.data.float_data.FloatData at 0x7feaac13ace0>,
  <geoh5py.data.float_data.FloatData at 0x7feaac13aad0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7feaac13aef0>,
  <geoh5py.data.float_data.FloatData at 0x7feaac13bbb0>,
  <geoh5py.data.float_data.FloatData at 0x7fea80eb99c0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7fea80eb9630>,
  <geoh5py.data.float_data.FloatData at 0x7fea80ebaa40>,
  <geoh5py.data.float_data.FloatData at 0x7fea80eb98d0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fea80eb96c0>,
  <geoh5py.data.float_data.FloatData at 0x7fea80eba110>,
  <geoh5py.data.float_data.FloatData at 0x7fea80ebab60>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7fea80eb8fd0>,
  <geoh5py.data.float_data.FloatData at 0x7fea80ebb400>,
  <geoh5py.data.float_data.FloatData at 0x7fea80ebb1f0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fea80ebafe0>,
  <geoh5py.data.float_data.FloatData at 0x7fea80ebb820>,
  <geoh5py.data.float_data.FloatData at 0x7fea80d40130>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7fea80d40070>,
  <geoh5py.data.float_data.FloatData at 0x7fea80d404f0>,
  <geoh5py.data.float_data.FloatData at 0x7fea80d402b0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fea80d400a0>,
  <geoh5py.data.float_data.FloatData at 0x7fea80d40d30>,
  <geoh5py.data.float_data.FloatData at 0x7fea80d40b20>]}

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).