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('3d44eada-c0c6-4d71-ad9c-1f3e8303c600'),
  '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 0x7f0f74b49250>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0fa044c590>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0f74b03d10>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0f74b0ac50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0f74b06050>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0f74b06390>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0f74af8dd0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f0fa0475150>]

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('3d44eada-c0c6-4d71-ad9c-1f3e8303c600'),
  '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 0x7f0f755d3450>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b49510>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b49890>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f0fa0468f10>,
  <geoh5py.data.float_data.FloatData at 0x7f0fa0443790>,
  <geoh5py.data.float_data.FloatData at 0x7f0fa045c110>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f0f74af8d10>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b03c50>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b03e10>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f0f74b03f50>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b0add0>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b0af90>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f0f74b06d90>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b06fd0>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b06e10>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f0f74b063d0>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b06210>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b06b50>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f0f74b065d0>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b0a2d0>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74af8e90>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f0fa044c2d0>,
  <geoh5py.data.float_data.FloatData at 0x7f0f74b49050>,
  <geoh5py.data.float_data.FloatData at 0x7f0fa04754d0>]}

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).