Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
1999-04-26
2001-02-20
Strecker, Gerard (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C324S345000, C342S357490, C701S213000, C702S005000
Reexamination Certificate
active
06191587
ABSTRACT:
FIELD OF INVENTION
This invention relates to an improved system for obtaining magnetotelluric geophysical data. In particular, this invention relates to obtaining magnetotelluric geophysical data using satellite signals for synchronizing sampling of the geophysical data and a system for analysing such data.
BACKGROUND OF INVENTION
Magnetotellurics
Magnetotellurics (MT) is a geophysical technique invented in the early 1950s, which utilizes naturally-occurring fluctuations of the earth's magnetic field to obtain an image of the earth's subsurface resistivity structure. The resistivity structure can be interpreted in geological terms and converted to an image showing subsurface rock types, thicknesses, structures, etc.
The fluctuations of the magnetic field arise from electric currents flowing in the earth's ionosphere. The changes in these current flows generate low-frequency electromagnetic waves, similar to radio waves, but at much lower frequencies. These waves are propagated around the earth by repeated reflections (“skips”) from the surface and the ionosphere. Both surface and ionosphere are far more conductive than the resistive atmosphere in between. The atmosphere is sandwiched between two “mirrors” and thus acts as a “waveguide” or conduit for electromagnetic waves.
These electromagnetic waves have very little curvature since the waves are relatively distant from its source. The situation may be likened to the waves caused by throwing a stone into the surface of a body of water. Close to the point of entry or source, the circular wavefronts are strongly curved. But at greater and greater distances, the circular wavefront becomes less and less curved over a small distance. Taken to the limit, i.e. in “the far field”, the wave front can be approximated by a straight line over short distances. When considered in three dimensions, an expanding spherical wavefront can be well approximated by a plane surface over a small region.
One advantage of MT is that the wavefront is almost always far from its source, and is a “plane wave” or equivalently is “in the far field”. In practical terms, this means that the signal arriving over a significant geographical area is the same at every location in the area. Again, an analogy from everyday experience is useful: a radio signal arriving on this side of the earth from Australia is experienced as arriving nearly simultaneously, and at the same strength almost everywhere over an area of a few hundred square kilometers.
Unlike radio signals from fixed and definite sources, the MT signals arriving at a specific location can arrive from any direction. The MT signal is thus omnidirectional. The signals from a specific direction may be “well-coupled” or “poorly-coupled” to subsurface targets. The same situation arises with radio antennas. The antenna has to be rotated into the position of maximum sensitivity to the signal from a specific direction. However, in MT, because the signal is omnidirectional, and the orientation of subsurface targets generally unknown, for the best and most general result, it is necessary to use a measuring configuration that is omnidirectional, and thus independent of relative orientations between source-sensors-target.
The “plane electromagnetic waves” arriving at a given measuring area are refracted normal to the surface of the earth. The electromagnetic wave then propagates downward, normal to the surface, losing energy as it does so. The rate of energy loss in the vertical direction is exponential and depends simultaneously on two things: the frequency of the wave, and the electrical resistivity of the earth. The higher the frequency, and the more conductive the earth, the more rapid the attenuation. Thus, a signal at a specific frequency may penetrate quite deeply into the earth where the conductivity is low, or the penetration may be quite limited if the earth is quite conductive.
By Faraday's Law, the changing magnetic fields of the electromagnetic wave cause electric currents to flow in the earth (telluric currents). By measuring at the surface one component of the earth's magnetic field and a component of the earth's electric field at right angles to the magnetic field, it is possible to compute earth resistivity, albeit only in a single direction.
The resistivity of the rocks of the earth's crust varies over a range of more than ten million to one. For example, dense crystalline rocks, such as granite, with little or no pore space and little or no included fluids can have resistivity of approximately 100,000 ohm-m. By contrast, rocks laid down as sediments in ancient oceans frequently preserve in their pore spaces some of the ancient sea water, chemically modified to brines. Brines contain salts, and salty water conducts electricity quite well. Hence, marine sedimentary rocks usually have low resistivities, of a few ohm-m.
In other words, mapping the subsurface resistivity structure of the earth is a type of proxy subsurface geological mapping. Hydrocarbons are usually found in marine sedimentary rocks. Wherever these rocks are covered by denser, more resistive rocks, MT may be used to obtain an image of the subsurface resistivity structure which can be interpreted in terms of the gross rock structure. This is the basic idea behind the use of MT in oil and gas exploration.
Geological facies changes can be strongly correlated with resistivity changes. For example, a sandstone channel (more resistive) may grade into a shale (less resistive). A longshore bar with coarser sediments than its surroundings may preserve this porosity/permeability differential (with corresponding resistivity differential) throughout geologic time. Likewise, near-reef sediments tend to be more resistive then those more distant from the reef, because the sediments near the reef contain a higher proportion of more resistive carbonate fragments derived from the reef by wave action.
High-density networks of MT soundings (3-D MT) can be used to map lateral subsurface conductivity changes within inverted depth ranges that correspond reasonably closely to specific geological units or groups. Plan maps of such lateral resistivity changes (horizontal conductivity gradients) at various depths or in general the horizontal or vertical gradients of any measured or derived MT parameter can be used for a type of proxy subsurface geological mapping, providing information which may assist on locating subsurface oil and gas, geothermal, metal or ground water deposits.
Stratigraphic traps are commonly associated with porosity/permeability boundaries which exhibit significant resistivity changes, Dickey, P. H. and Hunt, J. M. “Geochemical and Hydrogeologic Methods of Prospecting for Stratigraphic Traps” pp. 136-167 in AAPG Memoir No. 16 “STRATIGRAPHIC OIL AND GAS FIELDS—CLASSIFICATION, EXPLORATION METHODS AND CASE HISTORIES” 1972.
Most marine sedimentary rocks contain interstitial brines which exhibit strong variability in ion concentration (and thus strong variability in resistivity) due to variability in fresh water flushing. The saltier the brine, the lower its resistivity. Zones of high salt concentration are sometimes correlated with oil fields. Collins, A. G. “Oilfield Brines” pp. 139 ff in book “Developments in Petroleum Geology - 2” edited by G. D. Hobson, Applied Science Publishers, London, U.K., 1987 and Dickey, P. H. and Hunt, J. M. “Geochemical and Hydrogeologic Methods of Prospecting for Stratigraphic Traps” pp. 136-167 in AAPG Memoir No. 16 “Stratigraphic Oil and Gas Fields—Classification, Exploration Methods and Case Histories” 1972.
The frequently-reported “halo” effect above oil fields is variously ascribed to increased concentrations of metallic magnetic minerals which also have measurable resistivity differences.
Certain shallow heavy oil fields with thick associated brine sections are also suitable targets for resistivity mapping. Klein, J. H. “Spectral Induced Polarization Survey—David Field, Alberta, Canada” presented at 36th Annual Meeting of the Midwest Society of Exploration Geophysicists, De
Gowling Lafleur Henderson LLP
Strecker Gerard
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