2010101828
Sihle Maphanga
GHR613
Assignment 4 Electromagnetics survey systems. 1. Towed Streamer Electromagnetic System

Fig 1.2: Towed streamer electromagnetic survey system
PGS successfully completed a Multiclient Data Acquisition Program validating the newly developed Towed Streamer Electromagnetic System during October 2012. The Multiclient program was conducted in conjunction with pre-funded surveys. This is a significant milestone that marks the arrival of an effective, commercially viable and highly efficient new method for acquiring resistivity data to the market. See further details Electromagnetic Streamer development has been a major R&D focus within PGS since 2004, complemented by the acquisition of MTEM Ltd. in 2007. Successful field trials with prototype systems were completed over the Peon and Troll fields (North Sea) during 2009 and 2010. A further validation trial was conducted in 2011.The principle objective of this new approach to Controlled Source Electromagnetics (CSEM) is to provide:
Resistivity Surveys to enhance subsurface understanding, discoveries are becoming less obvious and fewer large fields are being found. Adding electromagnetic data to seismic can highlight prospective areas that may have been overlooked. Information – Intelligence having an additional attribute to use in the delineation and characterization of potential reservoirs can reduce risk and improve drilling success.

How it operates
The new Electromagnetic Streamer can be deployed from a seismic vessel and can acquire data at similar speeds to seismic operations. Already proven to be effective in water depths between 50m and 400m, the future development program will aim for deeper water and deeper targets. The possibility of simultaneous acquisition of resistivity data with seismic data is also being developed for future release.
2. THE EM 31 system
•Intercoilspacing of 3.7 m.
•Effective depth of exploration = 6 m (pole horizontal), 3 m (pole vertical)
•Detect layering by rasingand lower instrument.
•Procedure: Lay out survey line with a measuring tape, walk to measurement location, turn on transmitter read apparent conductivity.
APPLICATIONS
Finds voids & solution features in soil and rock Locates former mineshafts, crown holes & subsidence features and Detects bedrock discontinuities & mineralised veining

Fig 2: showing the EM31 survey system
SYSTEMS and OPERATION
During survey operations the EM system is suspended above ground, avoiding direct contact. This operational mode makes EM surveys rapid and cost effective compared to conventional resistivity surveys. The transmitting coil outputs a primary electromagnetic field, which induces a secondary field in the ground. The receiving coil measures the magnitude of the secondary field (quadrature component) and the ratio between primary and secondary fields (in-phase component). Quadrature fields are proportional to ground conductivity, responsive to bulk changes in lithology, groundwater and ground contamination. Metal produces strong secondary fields, so the in-phase component is a useful indicator of buried metal targets.
Data is collected as point readings taken at regular intervals along a survey grid set out over the site area. The spacing between grid-lines and reading stations is dependent upon the target size. Generally smaller targets require closer survey lines and denser spaced readings.
The data readings acquired during sitework operations are recorded on a digital data logger, for later downloading to a PC for post-survey processing and interpretation. The most commonly used interpretation procedure is contouring, carried out with specialist interactive software to produce contour plans.
The contoured data is analysed in detail by our experts to identify anomalous features relative to the general background. Once identified, the anomalies are correlated with local ground conditions. Survey results are presented as plans tied in to site co-ordinates, in a readily understandable engineering compatible format.
3. HUMMINGBIRD
Frequency Domain Electromagnetic Survey System (airborne system)
The depth to which a single EM frequency can penetrate in the Earth is a function of the frequency and the conductivity of the Earth (frequency x conductivity) among other things. Lower frequencies can penetrate deeper than higher frequencies, but higher frequencies are more sensitive to surficial conductors and subtle changes in the ground conductivity. Therefore, multiple frequencies are essential to maximize the value of an EM survey. The HUMMINGBIRD EM system measures the in phase “I” and quadrature “Q” (sometimes called out of phase). The amplitude of these components is always given as a value that is relative to the transmitted primary. In general terms, the ratio of in phase to quadrature (I/Q) depends mostly on the conductivity of the geology and the operating frequency, while the amplitude depends mostly on the depth of the conductor below the EM sensor Combined EM and mag surveys are a very powerful tool to locate and define many types of mineral deposits including gold, silver, copper, and diamonds. Additional sensors include a radar altimeter, barometric altimeter, differential GPS, and spectrometer. Precision’s frequency domain helicopter EM survey systems are integrated around the 5-frequency HUMMINGBIRD sensor. The multi-frequency and multi-coil EM system measures the in-phase and quadrature responses from multiple coil pairs in a tubular bird, towed beneath a low-flying helicopter. All components of the HEM instrumentation are digitally controlled. The HUMMINGBIRD is currently the only operating HEM system that is 100% digital from front to back. Data are transmitted from the sensors to the data acquisition console on board the helicopter using a lightweight serial cable. The helicopter also contains a colour LCD screen for in-flight monitoring, a removable PCMCIA hard disk, and a 3D pilot guidance system. Helicopter-borne geophysical surveys have proven to be useful for the discovery of a wide variety of mineral deposits. Electromagnetics combined with magnetics and radio metrics have accounted for the discovery of billions of dollars’ worth of mineral deposits, identified numerous ground water reservoirs, and provided immense volumes of geological and environmental data. Helicopter EM surveys are ideally suited for working in rugged mountain terrain, remote areas, and for the acquisition of high resolution data.
APPLICATIONS:
Good contouring characteristics in mountainous terrain, Fully digital, No compensation required, Accurate anomaly positioning (+/- 2m), Simultaneous determination of 5 frequencies ranging from 900 to 35,000 Hz, All digital samples are supplied as in-phase and quadrature measurements, Simultaneous magnetic and (optional) gamma ray spectrometer data collection, Integrated DGPS pilot navigation, Base & precious metals exploration, Geothermal mapping, Kimberlite exploration, Environmental engineering, Sand and gravel mapping, Ground water exploration, Shallow sea bathymetry, Sea-ice thickness mapping, Mapping of fresh water/salt water interfaces, Permafrost mapping and Geological mapping.

Fig3: humming bird

Fig 4: data interpretation of the humming bird
4. VTEM Versatile Time Domain Electromagnetic Surveying
Time domain airborne system
VTEM is the leading time-domain electromagnetic system in the world. The coincident, vertical dipole transmitter-receiver configuration provides a symmetric system response. Any asymmetry in the measured EM profile is due to the conductor dip, not the system or direction of flying. This allows for easy identification of the conductor location and for interpretation of the EM data. The low noise receiver, plus the high power transmitter yields a low noise system that has the best signal-to-noise ratio of any airborne survey system available commercially. Over 2,000,000 line-kms flown, Best signal-noise levels, System most often selected by clients to fly airborne EM surveys, Three diameters to select from based on the terrain and desired depth of penetration, Adjustable Dipole Moments 250,000 NIA over 2.5 Million NIA, Single mag or horizontal mag options adjustable pulse width.
Benefits of VTEM:
Designed to detect and discriminate between moderate to excellent conductors using a low base frequency, long pulse width, and derived B-field. The B-field is derived from integrating data collected at 192 kHz over the entire waveform. In-loop transmitter-receiver geometry to provide a symmetric response that allows for intuitive conductor interpretation. Low noise receiver and in-loop transmitter-receiver geometry provides for high spatial resolution. Low base operating frequency – standard is 30 or 25Hz time base operation in countries with 60 or 50Hz. Providing both high near surface resolution with as well as the depth of penetration
Data presentation
Recognizing the importance of providing results in a timely manner data compilation is done progressively during each survey, with preliminary maps available immediately after the flying is complete. Final data processing is completed at the office. The latest data processing tools and techniques are used to provide a wide range of final geophysical survey products such as maps, resistivity-depth sections, and data profiles.

Fig5: VTEM data presentation
5. Airborne Geophysical Surveys and Exploration ZTEM frequency domain air borne system
Leveraging the Earth’s natural or passive fields for plane wave frequency domain electromagnetic surveys is an innovative new airborne AFMAG (Audio-Frequency Magnetics) system for plane-wave frequency domain electromagnetic surveying, which can be operated by a helicopter or a fixed wing aircraft. Utilizing the naturally occurring or passive EM fields from worldwide thunderstorm activity of the Earth as the source of transmitted energy, this revolutionary technology has a proprietary receiver design and advanced digital electronics and signal processing. This allows for low noise levels and exceptional resolution and unparalleled depth penetration, well-suited to mapping deeply buried, porphyry hosted and structurally controlled target types.
Exceptional depth, resolution, and low noise plane wave frequency domain electromagnetic surveys.
Helicopter flown ZTEM is operated in a similar method as the well-known VTEM (Versatile Time Domain Electromagnetic) system, but rather then inducing a secondary field the way the VTEM Transmitter does, the ZTEM (Z-Axis Tipper Electromagnetic) system uses the natural or passive fields of the Earth’s worldwide thunderstorm activity as the source of transmitted energy. These natural fields are planar and due to the manner in which they propagate, are horizontal. Any vertical field response is caused by lateral conductivity contrasts in the Earth and is measured by the ZTEM.
The vertical EM field is remotely referenced to the horizontal base station coils positioned within the survey area. The proprietary receiver design, along with new advances made in modern digital electronics and signal processing, allows the ZTEM to acquire data at exceptionally low-noise levels. This makes the ZTEM an effective deep resistivity mapping solution, unique among airborne EM methods. Unparalleled resolution and depth in plane wave frequency domain electromagnetic surveys
Applications:
The ZTEM system serves as an excellent solution for mapping deeply buried, porphyry hosted and structurally controlled type targets. The result is that the ZTEM provides great resolution with unequal depth of investigation in terms of airborne electromagnetic measurements. ZTEM data are related to resistivity/conductivity mapping of the subsurface.
Passive EM technique – Source of the primary field signal is naturally occurring audio frequency magnetic fields from worldwide thunderstorm activity. Frequency range – “audio range” airborne AFMAG can operate from 30 to 720 Hz (depending on signal strength). Superior exploration depth – often exceeds 2000 metres from numerical simulations. Low frequency 30 Hz for penetration through deep or conductive cover. Excellent resistivity discrimination and detection of weak anomalies due to the nature of the natural EM fields. System is easily transportable- can be disassembled for packaging in relatively small units for shipping to surveys around the world.
6. Transient Airborne Electromagnetics the INPUT system
Historically, the most commonly encountered system of this type was the input system. The newer systems GEOTEM and MEGATEM (Fugro Airborne Surveys) function in a similar way to input, Thus for simplicity we will examine only the input system. The input system the transmitting coil, usually encircling a fixed wing aircraft, is energized by what is, essentially, a step current. In the absence of conductors, a sharp transient pulse proportional to the time derivative of the magnetic field is induced in the receiver. When a conductor is present, however, a sudden change in magnetic field intensity will induce in it a flow of current in the conductor which will tend to slow the decay of the field. Figure 3.1-2 illustrates this situation. The switching is repeated several times a second as the aircraft follows its flight line, so that the signal is virtually continuous.
The receiver "listens" only while the transmitter is "quiet" so that problems arising out of relative motion between transmitter and receiver, because the receiver is towed in a bird behind the aircraft, are virtually eliminated. Moreover, if the entire decay of the secondary field could be observed, the response would be equivalent to AC measurements made over the whole of the frequency spectrum. It is important to note in this connection, however, that not the decay function itself but only its time derivative can be recorded if a coil is used as the detector. This means that the anomalous fields which decay very slowly are suppressed in amplitude more than the others, and since these are the very ones generally associated with good conductors, there would seem to be an inherent weakness in this system. Because it is difficult to precisely synchronize the instant when the transmitter becomes "quiet" with the instant that the receiver begins to "listen", it is nearly impossible to record the entire function. This is equivalent to being unable to record many of the lower frequencies in the a-c spectrum. That should be noted, however, that in the past several years, significant progress has been made in measuring the early time response.

Figure 6: A sketch of the INPUT transient airborne EM system operation.
The primary field is a step function and the receiver records the decay of the field after the transmitter stops transmitting (Grant and West 1965). Typically, the time derivative of the decay function is measured using from six to twelve different time delays from the instant that transmitter stops transmitting before recording the signal received.
Applications:
Conductivities of geological materials range over seven orders of magnitude, with the strongest EM responses coming from massive sulphides, followed in decreasing order of intensity by graphite, unconsolidated sediments (clay, tills, and gravel/sand), and igneous and metamorphic rocks. Consolidated sedimentary rocks can range in conductivity from the level of graphite (e.g. shale’s) down to less than the most resistive igneous materials (e.g. dolomites and limestone’s). Fresh water is highly resistive. However, when contaminated by decay material, such Lake Bottom sediments, swamps, etc., it may display conductivity roughly equivalent to clay and salt water to graphite and sulphides.
Typically, graphite, pyrite and or pyrrhotite are responsible for the observed bedrock AEM responses. The following examples suggest possible target types and we have indicate the grade of the AEM response that can be expected from these targets. Massive volcano-sedimentary strata bound sulphide ores of Cu, Pb, Zn, (and precious metals), usually with pyrite and/or pyrrhotite. Fair to good AEM targets accounting for the majority of AEM surveys. Massive pyrrhotite-pentlandite bodies containing Ni and sometimes Cu and precious metals associated with noritic or other mafic/ultramafic intrusive rocks.
7. THE EM 34 system

Figure 6: showing the EM 34 method
Terrain conductivity measurements (also commonly referred to as Electromagnetic techniques) are characterized by the Geonics EM-34 and EM-31 instrumentation, although other systems are also available. EM surveys measure the apparent conductivity of the subsurface in mS/m (milliSiemens/meter). Variations in conductivity may indicate changes in composition, layer thickness, or moisture content, or the presence of buried metal such as drums, and/or contamination. EM-31 is used to measure the quadrature and in-phase component (in-phase very sensitive to buried metals). Penetration is typically up to 6 meters in normal subsurface conditions. Horizontal and vertical loop components are recorded with EM-34 using coil separations of 10, 20 and 40 meters. Using different coil separations allow for increased penetration.
The vertical component is also very sensitive to steeply dipping features such as water soaked fractures. In this configuration the system operates in the slingram mode where a steeply dipping conductor appears as an M-shape feature on the vertical component data.
EM surveys can be performed rapidly and large areas can be covered much quicker than gravity and resistivity surveys. Data can be acquired at single stations and/or continuously along lines. Data is collected on a grid or profile lines, depending on the target characteristics. Station spacing must not be larger than the dimensions of the expected target. Data can be acquired in profiling or sounding mode where sounding gives depth information. However, most EM surveys are conducted in profiling mode as lateral variations in conductivity are more of the interest in most environmental.
Applications
Buried metallic objects, Buried tanks, drums, utility lines, Mapping of landfills and trenches, Mapping of contaminant plumes, Mapping of groundwater, Mapping of fractures, faults, Mapping bedrock topography and Mineral exploration

8. ERA-MAX
Ground survey time domain survey system
ERA-MAX is a general-purpose electromagnetic instrument designed for electrical survey on any surface, eight types of input devices, noncontact and magnetic and methods of measurement
Applications:
•Engineering survey for construction and inspection of active pipelines, roads, hydraulic facilities
•Search and exploration of metallic and non-metallic mineral deposits
•cology, archeology
•Inspection of house footing and urban underground utilities
SURVEY METHODS
•Electrical exploration resistivity method (vertical electric sounding, profiling)
•Charge method
•Natural electric field method