Gravity

Potential Field Methods from Quantec Geoscience Quantec Geoscience is fully experienced in performing gravity ground surveys for various applications, including Oil & Gas Exploration, Geothermal Exploration, and Mineral Exploration. In this brief overview, we look at Oil & Gas Exploration and Mineral Exploration applications.

 

Overview of Gravity Methods

Gravitational measurements reflect the variation of mass distribution (or density) within the Earth. Locally, there are subtle variations that reflect density variations proximal to the Earth’s surface. It is these variations that we measure, map and interpret as part of the search for Oil & Gas resources.

Gravity surveys are generally carried out on the ground. In remote areas, helicopters are used to reach the sites, but the actual measurement is done by placing a gravimeter on the Earth's surface. Recently, gravity data have been acquired from aircraft, but the resolution is generally only suitable for reconnaissance surveys.

Interpretation of gravity data is performed on either profile or map data. Profiles are selected to run roughly perpendicular to geological structures, and a vertical cross-section is constructed that has a density distribution in accordance with the gravity anomaly observations. The shape of a particular gravity anomaly is usually indicative of the dip and depth extent of a geologic unit if there is a significant density contrast with the surrounding rock units. For example, over granite plutons, the extent of the gravity low (related to the lower density granites) can be use to accurately determine the shape of the pluton at depth and the dip angle of the contact with the surrounding rocks.

Map data are most often interpreted by the use of inversion techniques. Under certain assumptions (e.g. a constant thickness of the crust, or a constant density), a geological model that produces a gravity field that matches the observations can be calculated by inversion of the data. These types of models are often used to study large scale variations in crustal thickness or composition, for example along continental margins.

Applications of Gravity in Oil & Gas Exploration

In Oil & Gas Exploration, gravity has been used effectively for both reconnaissance and local geophysical mapping.  For example, on a local level, careful examination of rock densities can help to make inferences about reservoirs. Rock densities from limestones and sandstones are significantly different, giving a means of calibrating densities for further interpretation. On a reconnaissance scale, gravity results may indicate the presence of structure at depth which control Oil & Gas entrapment.

While reconnaissance remains an important goal, the nature of gravity work is also changing driven by advances in instrumentation, high-resolution survey acquisition strategies, improvements in processing and interpretation software, and integration with 3-D and other seismic data. Some of the newer, more locally focused applications include:

• Using gravity modeling to reduce expensive, model-driven, prestack depth migration
• Evaluating reef structures at the edge of basins
• Evaluating salt domes, including geometry and extents
• Identifying structure associated anomalies associated with fault blocks
• Resolving top of salt structures that may be invisible to seismic

Quantec Geoscience is fully qualified to conduct, process and interpret gravity surveys from either reconnaissance or local surveys based on its extensive experience with gravity throughout its history. The company operates proven Lacoste and Romberg model gravimeters which are known for their accuracy and repeatability. We also have the ability to perform quality control (5% repeat stations) and to apply all corrections such as Latitude, Free Air and Bouguer corrections -- providing high quality data for interpretation purposes in Oil & Gas applications.

Applications of Gravity in Mineral Exploration

In mineral exploration, the geology typically varies much more dramatically than in oil & gas exploration where targets are large and laterally extensive. Hence, reconnaissance surveys are not often done except to find the extent of larger structures that may affect exploration in an area.

More commonly, gravity surveys are focused or local surveys done with the objective of finding the excess mass related to massive mineral deposits (which affect the rock density and are distinguishable from surface). In addition, it may be desirable to map basement locally with local gravity surveys.

Interpretation of gravity surveys can be quite revealing with simple and complex model shapes used in the form of forward or reverse inverse models which guide the conceptualization of metallic mineralization and possible drill target recognition.

Notes on Quantec Gravity Instrumentation

Quantec uses LaCoste & Romberg Model G gravity meters manufactured in Austin Texas. The instrument has a worldwide calibration range and a reading resolution of 0.01 mgal. The L & R gravity meter is made of metal parts. Since metal parts creep when thermally expanded or contracted the meter is accurately thermostated.

The design of the meter allows it to be very sensitive to small changes in gravity. The simplified diagram of the meter shows a mass at one end of a horizontal beam. At the other end of the beam are a pair of fine wires and springs that act as a frictionless hinge for the beam. One purpose of the hinge springs is to help eliminate damage to the meter from all but the most severe impact.

The beam is supported from a point just behind the mass by a "zero length" spring. The spring is at an angle of approximately 45 degrees from horizontal. The meter is read by nulling the mass position, that is, adding or subtracting a small amount of force to the mass to restore it to the same "reading" position. This is accomplished by lifting up on the top end of the zero length spring. This must be done with great accuracy and is accomplished with a series of levers. In turn, the levers are moved by a high precision screw which in turn is rotated by a gear box with considerable reduction.

The lever system and screw are accurately calibrated over their entire range. Calibration factors depend only on the quality of the lever system and measuring screw, not upon a weak auxiliary nulling spring as are used in other meters. For this reason the calibration factors of the L & R meters do not change over time.

Why Choose Quantec as Your Gravity Services Partner?

What makes Quantec unique as a Gravity services partner are attention to high quality data, cost and safe operating procedures. Quantec’s specially trained and experienced operators are a key component to acquiring the best quality data possible. These operators quickly recognize issues in the field and can deal with these effectively. Cost considerations are also key as Quantec has proven in the past. Moreover, Quantec is conscious of the need to maintain measurement sites in an environmentally acceptable manner and operator safety as two components of its safe operating policy. When it’s time to consider a partner for your next ground survey, Quantec is the group to which to turn.