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('4d6a325e-34e5-4a49-92ac-050341c6da75'),
  '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 0x7f7a69dbfb50>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f7a69e5d930>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f7a69e5e8c0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f7a69e5ff40>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f7a69d187c0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f7a69e5fa90>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f7a69e5ebf0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f7a69d18940>]

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('4d6a325e-34e5-4a49-92ac-050341c6da75'),
  '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 0x7f7a95f06320>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69dbe4d0>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69dbd5d0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f7a69dbd960>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69dbfd90>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69e5d870>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f7a69e5c4f0>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69e5dd50>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69e5e8f0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f7a69e5ea70>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69e5f670>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69e5ff70>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f7a69e5ffd0>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69d18670>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69d187f0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f7a69d186a0>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69d18a30>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69d19030>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f7a69e5f580>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69e5fc40>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69e5c040>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f7a69e5eb60>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69d190c0>,
  <geoh5py.data.float_data.FloatData at 0x7f7a69d19210>]}

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).