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.

from pathlib import Path

import numpy as np

from geoh5py.objects import MTReceivers
from geoh5py.workspace import Workspace


# Create a new project
h5file = Path("my_project.geoh5")
workspace = Workspace(h5file) if h5file.exists() else Workspace.create(h5file)

# 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(0.0, 20.0, 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 geoh5py.objects.surveys.electromagnetics.magnetotellurics.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('5ba45e87-aaaf-49bc-8a0f-de07c2d293c1'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Channels

List of frequencies at which the data are provided.

mt_survey.channels = [1.0, 10.0, 100.0]

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 module-geoh5py.groups.property_group.PropertyGroups defining the various data components (e.g. Zxx (real), Zxy (imag), …). It is not required to supply all components of the impedance 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 geoh5py.objects.surveys.electromagnetics.base.BaseEMSurvey.add_components_data() can help users add data from nested dictionaries. Below is an example using four components:

# Arbitrary data generator using sine functions
def data_fun(c, f):
    return (c + 1.0) * np.sin(f * np.pi * (x_loc * y_loc).ravel() / 200.0)


# Create a nested dictionary of component and frequency data.
data = {
    component: {
        f"{component}_{freq}": {
            "values": (ff + 1) * 1000.0
            + (cc + 1) * 100.0
            + 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 0x748b70086a20>,
 <geoh5py.groups.property_group.PropertyGroup at 0x748b224a2bd0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x748b224a34a0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x748b224a3da0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x748b2231c6e0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x748b237acda0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x748b2231d820>,
 <geoh5py.groups.property_group.PropertyGroup at 0x748b2231e120>]

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('5ba45e87-aaaf-49bc-8a0f-de07c2d293c1'),
  'Survey type': 'Magnetotellurics',
  'Unit': 'Hertz (Hz)'}}

Data channels associated with each component can be quickly accessed through the geoh5py.objects.surveys.electromagnetics.base.BaseEMSurvey.components property:

mt_survey.components

{'Zxx (real)': [<geoh5py.data.float_data.FloatData at 0x748b2243e3c0>,
  <geoh5py.data.float_data.FloatData at 0x748b6019bad0>,
  <geoh5py.data.float_data.FloatData at 0x748b225a00b0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x748b23766b40>,
  <geoh5py.data.float_data.FloatData at 0x748b2243d9d0>,
  <geoh5py.data.float_data.FloatData at 0x748b224a2630>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x748b2261b410>,
  <geoh5py.data.float_data.FloatData at 0x748b224a3200>,
  <geoh5py.data.float_data.FloatData at 0x748b224a3350>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x748b224a1f70>,
  <geoh5py.data.float_data.FloatData at 0x748b224a3b00>,
  <geoh5py.data.float_data.FloatData at 0x748b224a3c50>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x748b224a3380>,
  <geoh5py.data.float_data.FloatData at 0x748b224a3e00>,
  <geoh5py.data.float_data.FloatData at 0x748b2231c590>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x748b224a3c80>,
  <geoh5py.data.float_data.FloatData at 0x748b2231cd40>,
  <geoh5py.data.float_data.FloatData at 0x748b2231ce90>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x748b2231c5c0>,
  <geoh5py.data.float_data.FloatData at 0x748b2231d610>,
  <geoh5py.data.float_data.FloatData at 0x748b2231d6d0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x748b2231cce0>,
  <geoh5py.data.float_data.FloatData at 0x748b2231df10>,
  <geoh5py.data.float_data.FloatData at 0x748b2231dfd0>]}

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).

workspace.close()