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.

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.

mt_survey.metadata

{'EM Dataset': {'Channels': [],
  'Input type': 'Rx only',
  'Property groups': [],
  'Receivers': UUID('43a68a28-6159-4e50-afe2-ab2b0cc43766'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Channels

List of frequencies at which the data are provided.

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:

# 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)

[<geoh5py.groups.property_group.PropertyGroup at 0x7f8fe4c8e800>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8fc12b68f0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8fc12b6530>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8fc12b70a0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8fc12b78e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8fc12b7fa0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8fc11309d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f8fc1131210>]

Metadata are updated immediately to reflect the addition of components:

mt_survey.metadata

{'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('43a68a28-6159-4e50-afe2-ab2b0cc43766'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Data channels associated with each component can be quickly accessed through the BaseEMSurvey.components property:

mt_survey.components

{'Zxx (real)': [<geoh5py.data.float_data.FloatData at 0x7f8fe4c8f580>,
  <geoh5py.data.float_data.FloatData at 0x7f8fe4c8fc70>,
  <geoh5py.data.float_data.FloatData at 0x7f8fe4c8f910>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f8fe4c8e410>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b5510>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b5900>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f8fc12b5630>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b5390>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b6860>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f8fc12b69b0>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b57b0>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b5840>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f8fc12b6b00>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b7340>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b7130>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f8fc12b6f20>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc12b7b80>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc1130100>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f8fc1130190>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc1130430>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc11301f0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f8fc1130160>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc1130c70>,
  <geoh5py.data.float_data.FloatData at 0x7f8fc1130a60>]}

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).