Movinmarine

WHAT IS THE “RADAR M2”?

Radar M2TM is the system created by Movin Marine for the remote sensing geological prospection. It combines a range of sensors for the data gathering.

Introduction to remote sensing

Remote sensing is the ability to measure something without physically touching it. Previously developing remote sensing techniques, the only way to obtain such measurements required a direct contact, we have;

1) Variations on terrain elevation (like mountain range) and geological composition (rock density variations) result in microvariations on earths gravitatory field. As an example, ocean Ridge results in a measurable protuberance in the ocean level. Gravitational field maps are adjusted according to topography and rock density, the resulting maps display corrected gravity field. Dense material (like basalt rocks) cause a significant increase in the local gravity anomaly, just like mountain range or positive orography, in contrast, sea trench and less dense rocks (like sedimentary ones) cause negative gravity field anomalies.

2) According to the physics laws: “Any existing substance over absolute zero degree (-273. 15oC) emits some kind of electromagnetic radiation”. This means that using the adequate receiver, we can detect any object and discriminate it from any other. In the earth sciences field, remote sensing refers to the ability of satellites to detect electromagnetic radiation (EMR) from earth’s surface or atmosphere1. Taking Sun as an example, it dissipates an enormous quantity of energy directly towards space. When this energy reaches Earth, it can be observed in the visible spectrum, like white light but there are some others like microwaves, x-ray, infrared light, etc. that can also be measured. For us to be able to detect through remote sensing, the media through the EMR propagate must be transparent to those EMR. Figure 1 shows atmosphere opacity to suns EMR.

radar m2
Figure 1: Earths atmospheric opacity. From NASA (modified) – public access.

As seen in Figure 1, the spectrum region corresponding to the radio waves is the most permeable in the atmosphere. This region is labeled as the microwave band and is the region in which applications like Wi-Fi, cell phone, radio, television, Bluetooth, radar, remote control and other similar remote controlled devices operate.

On the other hand, gravity variations are measured with special devices named gravimeters. A gravimeter is an accelerometer especially designed to measure gravity. Even when design is the same as other accelerometers, gravimeters are designed to measure minimum changes in earth’s gravity caused by changes in rock’s density, by earth’s shape and tide variations. Gravimeter measurements are taken in “Gal” units where 1 Gal is defined as 1 centimeter per square second (1 cm/s2). Airborne gravimeters are a subgroup of portable gravimeters: stabilizers are required in order to isolate the device from the aircraft acceleration and a post process to clear high frequencies noise.

M2TM radar is built on the idea to combine both principles to attain an optimal result.

Remote sensing with microwaves

There is a wide range of microwave bands, depending on frequency and wavelength its application changes. This application is strongly related with the depth penetration of the wave. In table 1, the most commonly underground used bands.

Band Wavelength (cm) Frequencies (MHz)
VHF 1000 - 100 30 - 300
P (UHF) 100 - 30 300 - 1000
L 30 - 15 1000 - 2000

Table 1: Microwave bands most commonly used in mineral prospection.

Remote sensors which use electromagnetism may be classified according to being active or passive. In this case only one sensor is applied.

Radiometers; don’t create or irradiate any kind of signal or stimulus. Passive sensors detect energy radiated from natural sources (mainly Sun and earth) or energy radiated from artificial (like Wi-Fi, TV, remote door locks, etc.). Cameras and passive radars are examples of passive sensors.

Physical properties measurable with microwaves

Passive sensors are best for the detection of physical properties of the observed object. An optical camera gives us information about the color range (properties of the Surface of the material) and a thermal camera gives temperature of the objects present in the observed scene.

For the most part, using microwaves changes in electrical conductivity, thermal and dielectric constant of the underground may be detected.

a) One of the best examples of passive radar able to measure the electrical properties of the underground is the device on board of the European Space Agency1 SMOS satellite. Uno de los mejores ejemplos de radar pasivo capaz de medir las. As indicated by its acronym Soil Moisture and Ocean Salinity this mission measures sea salt concentration and soil humidity using the electrical properties.

b) GPR (Ground Penetrating Radar), is the best example of radar that allows the creation of a dielectric constant map of the underground applying broadband antennas. It’s an active radar and only receives and processes waves reflected with source its emitter device. The advantage being through filters is extremely easy to remove recorded interferences. Nevertheless, its penetration is very shallow.

In figure 2 passive radar device’s working scheme is shown. Allows the detection thanks to changes due to the presence of an element with physical properties very different to its surroundings.

In the case of passive radars, some relevant aspects must be considered in order to adequately process received data.

The most relevant effects most commonly affecting received data are; variations on their properties due to temperature and interferences due to external sources-generated emissions, usually due to artificial sources.

In the first case, effects of temperature on data may be mitigated through use of hyperspectral sensors like the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) from NASA2

Remote sensing with microgravimetry

EThe device Radar M2TM applies a referential inertia gravimeter. This gravimeter consists of three accelerometers of very high precision and two gyroscopes mounted in an enclosed temperature-controlled environment. Accelerometers are completely isolated from aircraft’s movement through three servomotor-controlled plaits to compensate the aircraft’s movement. The system is designed to allow the accelerometers to be aligned with the gravity vector. Gyroscopes’ drift is tracked and lately corrected. GPS data is used for both aircraft precise localization and kinematic and Coriolis accelerations corrections. Finally, gravity data are filtered to eliminate high frequency noise from the aircraft.

The gravimetric methodology consists on the measurement of gravity’s acceleration on a predefined point net in the field with the objective to detect variations in underground geological unit densities, actual gravimeters are extremely precise tools (about 0.001 mGal) and overall weights of less than 2.5 Kg.

Applications

  • Studies in sedimentary basins: Oil & mineral explorations.
  • Underground Waters exploration.
  • Geodesy.
  • Regional Geological mapping.
  • Civil engineering – Geotechnical studies.
  • Archaeology.

On the other hand, gravimeters have a major disadvantage and it is the fact that they are more sensitive to the working environment, this is due to the fact the measurement parameters are based in completely mechanical systems thus being gravity pull is compensated by a measured mechanical force. This mechanical force is extremely sensitive to pressure and temperature due to stretching and widening, etc.

Therefore, thermal control in gravimeters is of very high importance since any change significantly affects data gathering because any contraction or expansion of any of the many parts composing microgravimeter may modify the outcome position of the weight, because of this one of the greatest precautions taken when manufacturing a gravimeter is that sensitive mechanism would be perfectly isolated, on general terms gravimeters are controlled with thermostats with 0.002ºC precision (Figure 2).

Distance laser

Differences in terrain result small changes in gravimeter measured data. The use of an alternate altimeter radar helps discriminating the origin of those changes are due to terrain composition or changes in terrain (Figure 2).

Thermal camera

A thermal camera is applied as a complement to measure temperature and infrared emissivity of soil. This allows to get lithological information of soil surface. This is the principle of measurements used in various publications of the ASTER sensor of Nasa’s Terra satellite (Figure 2).

Figure 2: Radar M2TM operational schematics
Figure 2: Radar M2TM operational schematics.

RADAR M2 Description

RADAR M2 is the combination of passive systems. It has integrated a passive microwave radar, a precision gravimeter and a thermal camera. The radar uses microwave frequencies in order to detect depth within underground. Considering presence of metallic parts and particles alters the electrical properties of the terrain. Anyways, measurement of those deep strata layers is obviously affected by the ones on top of them. To be able to discriminate between those measurements, RADAR M2TM (Figure 3) integrates a high precision gravimeter. Finally, a thermal imaging camera compensates ground temperature distribution with the data measurements taken by the passive radar from the surface to the maximum depth achieved.

Figure 3: Main parts of RADAR M2
Figure 3: Main parts of RADAR M2

Measurements location must be recorded with a non-inertial method, that’s why satellite GPS must be set. Aircraft’s inertial acceleration is compensated with the application of highly advanced compensation systems altogether with special filters during post-process.