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('4c89a6a2-9932-46ab-937e-31a00a57a518'),
  '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 0x719ecefceab0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x719ecefce690>,
 <geoh5py.groups.property_group.PropertyGroup at 0x719ecefcfd70>,
 <geoh5py.groups.property_group.PropertyGroup at 0x719ecee55190>,
 <geoh5py.groups.property_group.PropertyGroup at 0x719ecefcdd60>,
 <geoh5py.groups.property_group.PropertyGroup at 0x719ecefce8d0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x719ecefcf260>,
 <geoh5py.groups.property_group.PropertyGroup at 0x719ecf0d44a0>]

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('4c89a6a2-9932-46ab-937e-31a00a57a518'),
  '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 0x719ecf90fbc0>,
  <geoh5py.data.float_data.FloatData at 0x719ecf0767e0>,
  <geoh5py.data.float_data.FloatData at 0x719f10eda2d0>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x719ed01cbda0>,
  <geoh5py.data.float_data.FloatData at 0x719f10edb560>,
  <geoh5py.data.float_data.FloatData at 0x719ecefcf200>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x719f201875f0>,
  <geoh5py.data.float_data.FloatData at 0x719ecef9fd10>,
  <geoh5py.data.float_data.FloatData at 0x719ecefcffe0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x719ecefcecf0>,
  <geoh5py.data.float_data.FloatData at 0x719ecefce0c0>,
  <geoh5py.data.float_data.FloatData at 0x719ecee54f20>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x719ecefcfce0>,
  <geoh5py.data.float_data.FloatData at 0x719ecee559d0>,
  <geoh5py.data.float_data.FloatData at 0x719ecefcdbb0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x719ecee55070>,
  <geoh5py.data.float_data.FloatData at 0x719ecf0d60f0>,
  <geoh5py.data.float_data.FloatData at 0x719ecefce7e0>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x719ecee55f40>,
  <geoh5py.data.float_data.FloatData at 0x719ecefcef00>,
  <geoh5py.data.float_data.FloatData at 0x719ecefcf140>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x719ecefceae0>,
  <geoh5py.data.float_data.FloatData at 0x719f10edaf00>,
  <geoh5py.data.float_data.FloatData at 0x719ecefcfc20>]}

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()