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('047aedba-965b-48d0-9236-f8c6393a0d0f'),
  '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 0x7f6d8967c6d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f6d8960d5d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f6d8960de50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f6d8960ea90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f6d89611c50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f6d89613ed0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f6d89616e50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f6d8961b150>]

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('047aedba-965b-48d0-9236-f8c6393a0d0f'),
  '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 0x7f6d8967c510>,
  <geoh5py.data.float_data.FloatData at 0x7f6d8967c290>,
  <geoh5py.data.float_data.FloatData at 0x7f6d8967c610>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f6d8967cf90>,
  <geoh5py.data.float_data.FloatData at 0x7f6d8960d750>,
  <geoh5py.data.float_data.FloatData at 0x7f6d8960d950>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f6d8960de90>,
  <geoh5py.data.float_data.FloatData at 0x7f6d8960dd90>,
  <geoh5py.data.float_data.FloatData at 0x7f6d8960dbd0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f6d8960d150>,
  <geoh5py.data.float_data.FloatData at 0x7f6d8960e890>,
  <geoh5py.data.float_data.FloatData at 0x7f6d8960e950>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f6d8960ec90>,
  <geoh5py.data.float_data.FloatData at 0x7f6d89611a50>,
  <geoh5py.data.float_data.FloatData at 0x7f6d89611b10>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f6d89611e10>,
  <geoh5py.data.float_data.FloatData at 0x7f6d89613cd0>,
  <geoh5py.data.float_data.FloatData at 0x7f6d89613d90>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f6d89613f90>,
  <geoh5py.data.float_data.FloatData at 0x7f6d89616dd0>,
  <geoh5py.data.float_data.FloatData at 0x7f6d89616f90>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f6d89619090>,
  <geoh5py.data.float_data.FloatData at 0x7f6d89619fd0>,
  <geoh5py.data.float_data.FloatData at 0x7f6d89619f10>]}

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).