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('78613638-cf14-4db4-8193-efb6399afd4f'),
  '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 0x7fbe8c9e7430>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fbe9c580dc0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fbe7231d090>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fbe7231e380>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fbe7231ebc0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fbe7231f400>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fbe9c5820b0>,
 <geoh5py.groups.property_group.PropertyGroup at 0x7fbe7231f8e0>]

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('78613638-cf14-4db4-8193-efb6399afd4f'),
  '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 0x7fbe9c581ed0>,
  <geoh5py.data.float_data.FloatData at 0x7fbe8c9e7670>,
  <geoh5py.data.float_data.FloatData at 0x7fbe9c563a90>],
 'Zxx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fbe726a9510>,
  <geoh5py.data.float_data.FloatData at 0x7fbe9c562f20>,
  <geoh5py.data.float_data.FloatData at 0x7fbe9c580df0>],
 'Zxy (real)': [<geoh5py.data.float_data.FloatData at 0x7fbe9c581de0>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231cb80>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231d1e0>],
 'Zxy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fbe7231de70>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231d000>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231cf70>],
 'Zyx (real)': [<geoh5py.data.float_data.FloatData at 0x7fbe7231d210>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231e620>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231e410>],
 'Zyx (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fbe7231e200>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231ee60>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231ec50>],
 'Zyy (real)': [<geoh5py.data.float_data.FloatData at 0x7fbe7231ea40>,
  <geoh5py.data.float_data.FloatData at 0x7fbe9c580fa0>,
  <geoh5py.data.float_data.FloatData at 0x7fbe9c581cf0>],
 'Zyy (imaginary)': [<geoh5py.data.float_data.FloatData at 0x7fbe726a8460>,
  <geoh5py.data.float_data.FloatData at 0x7fbe9c580700>,
  <geoh5py.data.float_data.FloatData at 0x7fbe7231fa30>]}

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).