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.objects import MTReceivers
from geoh5py.workspace import Workspace


# 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(0.0, 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 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('8eeaa140-e010-4e13-93d5-b2d919c3eca3'),
  '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 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
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 0x7f038016bf40>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f038bdc1de0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f038bde9990>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f037fd4a200>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f037fd4aa70>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f037fd4b2b0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f037fd4baf0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7f037fd4b1c0>]

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('8eeaa140-e010-4e13-93d5-b2d919c3eca3'),
  '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 0x7f038bde98a0>,
  <geoh5py.data.float_data.FloatData at 0x7f038bdc34c0>,
  <geoh5py.data.float_data.FloatData at 0x7f038bde9660>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f037fd491e0>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd48730>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd48760>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7f037fd49120>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd49840>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd49810>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f037fd49c30>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd49480>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd4a140>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7f037fd4a4a0>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd49990>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd4a9b0>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f037fd4ad10>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd4a110>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd4b1f0>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7f037fd4b550>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd4a980>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd4ba30>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7f037fd4bd90>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd4bb20>,
  <geoh5py.data.float_data.FloatData at 0x7f037fd64280>]}

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()