Gravity Contour Research Articles

Aeromagnetic and gravity surveys in the Coso Range, California

The effect of an underlying magma reservoir cannot be identified within the complex gravity pattern in the Coso Range, California. Rather, linear gravity contours, which suggest a regional tectonic origin, enclose the location of most of the volcanic activity of the Coso Range. Faults along the edges of northwest trending, magnetic blocks probably provided paths of minimum resistance to the ascending viscous magma that was extruded as rhyolite domes. Dense, magnetic rocks associated with a complex mafic pluton 9 km in diameter form a relatively impermeable north border of the Pleistocene volcanic field. A heat flow high nearly coincides with the west half of a 6‐km‐diameter magnetic low. A 2‐km‐diameter outcrop of a pre‐Cenozoic silicic pluton, which has low magnetization compared to the surrounding metamorphic rocks, presumably typifies the rocks that underlie the magnetic low and heat flow high. Hydrothermal fluids may have destroyed some magnetite in the more magnetic wall rock, further reducing the magnetic intensity.

Regional gravity survey, northern Marysvale volcanic field, south-central Utah

New gravity data in south-central Utah reveal anomalies over major structural and volcanic features in the northern Marysvale volcanic field. Gravity lows are associated with the Mount Belknap and Big John calderas in the Tushar Mountains, and with the Red Hills caldera in the Antelope Range. Steep gravity gradients mark the locations of the Sevier, Elsinore, Dry Wash, and Tushar faults; gravity lows are observed over the grabens dropped down between these major normal faults. The steep gravity gradient across the Basin and Range–Colorado Plateau transition zone is not necessarily the result of deepening of the Moho, but may simply reflect the thick low-density volcanics in the Marysvale field as well as changes in the density of sedimentary rocks across the Cordilleran hingeline. East-northeast–trending gravity contours follow closely the northern edge of a series of Tertiary intrusive bodies and are generally aligned with the trend of the Wah Wah–Tushar mineral belt of southwestern Utah.

RECONNAISSANCE GRAVITY SURVEY OF AUSTRALIA

The Australian Bureau of Mineral Resources, Geology and Geophysics, completed a 15 year systematic reconnaissance gravity survey of Australia in 1974. Using helicopters, gravity stations were established at 11 km spacing over most of the continent, and at 7 km spacing in South Australia and Tasmania. Station elevations were measured barometrically and station positions were marked on aerial photographs and transferred to 1:250,000 photocenter base maps. Gravity and elevation controls were maintained by ties to specially established control networks. A Bouguer anomaly map of Australia has been compiled from the reconnaissance data, and from information from other gravity surveys by state governments, petroleum exploration companies, and academic institutions which together cover about 15 percent of the total area. The compilation uses a total of about 170,000 gravity observations. The accuracy of Bouguer anomaly values, taking account of errors in the gravity, elevation, and position measurements, is estimated to be better than ± 2.0 mgal. The Bouguer anomaly fields over Australia are divided into nine regional gravity divisions, in each of which the gravity contour pattern has some degree of uniformity, or is such as to imply tectonic affinities between the sources of individual gravity features. The gravity divisions can be correlated with various metamorphic complexes and orogenic provinces, ranging in age from Archaean to Paleozoic, which also form the basements to extensive sedimentary platform covers. The reconnaissance gravity results have been made public as the survey has progressed to assist in regional geologic studies and the search for petroleum and minerals. They have proved to be of considerable benefit in delineating regional structures and in providing leads for more detailed geophysical investigations. Predictions of the structures of sedimentary basins have been made and possible extensions to mineral provinces have been indicated. The results have also been used in deep crustal and upper mantle studies and in geodesy.

Inference of Granitic Batholiths by Gravity Studies in South India

Abstract Three well-defined gravity 'lows' in Kodaikanal, Gudalur and Erode regions are explained as due to granitic batholiths at depth. The scattered outcrops of granitic bodies exposed in these regions are suggested to be interconnected at depth, and, therefore, are merely the surface expressions of the three concealed batholiths. Although the exposures of the individual granitic bodies in the Kodaikanal-Coimbatore region are elongated in an east-west direction, it is surmised, from structural evidences, that the main batholith is elongated in a roughly NNW-SSE direction. A striking correspondence between the isoseismals for the Coimbatore earthquake of February 8, 1900 and the gravity contours is noticed. It is suggested that the Coimbatore earthquake (and the reported seismicity near Kottayam and 'Palghat Gap') could be the result of some minor readjustments of the boundaries of the batholith along fault planes.

Ambiguity assessment of gravity interpretation for an inhomogeneous multilayered sedimentary basin

Ambiguity in the gravity response of an earth model with transition layers has been assessed by calculating the channel capacity of the transmitter (transition layer). The channel capacity is used to determine the number of false gravity anomalies in a gravity contour, which acts as an index to the ambiguity range. The lower limit of this ambiguity range is fixed by using Gabor's modified formula, the beats produced between signal and noise being taken into account. The, upper limit is obtained by the Hartley-Tuller-Shannon formula after Sax (1966). Wider effective applications of this combined approach to various problems of interpretation have been indicated by the illustration of the controversial gravity interpretations of Mono basin in California.

The Oxfordshire Coalfield

THE discovery of eight coal seams of workable quality in the Geological Survey Apley Barn Borehole1,2, sunk in 1960–61 near Witney, prompted the recent sinking of a second borehole (1970–71) near Steeple Aston, Oxfordshire. This borehole is situated between Bicester and Chipping Norton some 14 miles north-east of the Apley Barn Borehole (Fig. 1). The exact site is 825 yards (750 m) at 254° from St Peter's Church, Steeple Aston (National Grid Reference SU 4687 2586). The geophysical Bouguer anomaly gravity contour map shows that the borehole lies in the eastern part of a low density area where a thick sequence of light sedimentary rocks could be expected. It was thought possible that this anomaly could be due to a thickening of the Coal Measures sequence as a whole and that perhaps even Middle Coal Measures (with valuable coal seams) could be developed. In the event, however, this hope has not been realized, and although Upper Coal Measures were proved the borehole has shown that the low density anomaly feature must be due to a thickening of Devonian and/or older sediments.

Crustal structure studies in the Bay of Biscay

A gravity contour map of the Bay of Biscay and the results of a recent seismic refraction survey in the area are discussed. It is shown that the crustal structure in the centre of the Bay is oceanic and that a large sediment-filled trough exists off Northern Spain, the latter possibly representing an ancient trench.

Regional Gravity Survey of the Northern Great Salt Lake Desert and Adjacent Areas in Utah, Nevada, and Idaho

From 1957 to 1961 a regional gravity survey was made over the northern part of the Great Salt Lake Desert and adjacent areas in Utah, eastern Nevada, and southeastern Idaho. A total of 1040 stations were taken over an area of about 7000 square miles. The results were compiled as a Bouguer gravity anomaly map with a contour interval of 2 mgal. The Bouguer values ranged from a high of about —120 mgal over the outcrop areas to a low of about —196 mgal over the alluvium-covered graben areas. The gravity high over the Raft River Mountains apparently corresponds with the Raft River Mountains anticline. A belt of gravity contours, with a total relief of 15–20 mgal, extending for 40 miles between the Wildcat Hills and the southern part of the Grouse Creek Mountains and beyond, is interpreted provisionally as caused partly by abrupt thickening and/or downwarping of the rocks of late Paleozoic age in a general northwestward direction, perhaps to form a foredeep (part of the Butte-Deep Creek trough of Steele, 1960) south of the Raft River Mountains; however, this anomaly could also be partly caused by overthrusting. Many Basin and Range faults, grabens, and horsts are indicated by the gravity data. In the northern part of the surveyed area, Junction Valley, Upper Raft River valley, and Curlew Valley are indicated to be grabens. The Newfoundland Range, in the northeastern part of the Great Salt Lake Desert, is a horst flanked by a graben on each side. The northwestern margin of the Great Salt Lake Desert comprises a complex pattern of Basin and Range fault blocks, large and small, that lie along a generally northward-trending belt or zone. The Silver Island-Pigeon fault block, which comprises the Silver Island Mountains, the Little Pigeon Mountains, and Pigeon Mountain, forms an elongate, arcuate horst that is flanked by a belt of grabens on the west (Pilot Valley, Lucin, and Grouse Creek grabens) and east (Wendover, Crater Island, Little Pigeon, and Pigeon grabens). A major fault zone is indicated along the east margin of the Pilot Range. The Pilot-Grouse Creek rift belt, at least 90 miles long, extends northward between Pilot Valley and the Upper Raft River valley and constitutes a major lineament in the earth9s crust along which the graben blocks were displaced downward relative to the adjacent mountain blocks. In the southern part of the rift belt, the grabens are separated by blocks (the Lemay and Lucin horsts) that were probably downfaulted relative to the large mountain blocks, but became lodged at intermediate height; in the northern part, however, the blocks (for example the Junction Valley graben), apparently merely broke from the main crustal unit along this belt of weakness. The indicated thickness of the valley fill of Cenozoic age in some of the graben areas ranges up to about 6000 feet.

Gravity Measurements over the Rhum Tertiary Plutonic Complex

DURING the past two years, sixty-six new gravity stations were established, using the Imperial College Worden gravity meter, on the islands of Rhum, Eigg, Canna and Sanday in the Inner Hebrides. Local bases on the islands were linked to Pendulum House, Cambridge, through the Mallaig gravity base established by Bullerwell and Phemister (private communication). The results were reduced to the International Gravity Formula, and after correction for terrain effects, the Bouguer anomalies were plotted as a gravity contour map (Fig. 1).

THREE DIMENSIONAL GRAVITY SURVEY*

ABSTRACTThe paper describes and discusses the results of an experimental gravity survey which was carried out underground on different levels of a mine, in the mine shafts, and on the surface above the mine workings.The paper is composed of three complementing sections. The part dealing with gravity measurements in the shafts gives also attention to the particular problem of the terrain corrections underground, due to the surface topography. The interval densities from gravity measurements in the shafts are computed and adjusted in accordance with known geology and compared with the stratigraphical columns of the shafts. The effect of the ore body on the stations in the shaft is derived theoretically and compared with the observed one.The gravity contours are constructed on different levels in the mine workings and discussed in relation to the known extent of the ore body. The gravity profile across a fault underground is presented and discussed. Another gravity profile was run underground in the same plan position as a surface traverse 1000 feet above it. The line of boreholes along this traverse gives good account of geology which includes step‐faulting. This known geology is compared with the deductions based on the gravity results. This is also done in the case of another gravity profile run over a known geological section. A number of gravity measurements were also taken in the same plan position, separated by the vertical distance of 800–1,100 feet. These points were placed by the boreholes previously drilled in the area. Attempt in correlation of these and gravity results is made.The densities computed from the gravity measurements are compared with the laboratory determinations of the densities, carried out on samples from different parts of the mine.The contours on the top of the base formation are constructed from the information obtained from the boreholes, and are compared with the gravity contours on the surface above.A simple method of computation of the effects of slabs and blocks is presented as applied to the calculation of the corrections for underground drifts and faults. A table is appended for use with this method.

GRAVITY‐METER SURVEY OF THE KETTLEMAN HILLS‐LOST HILLS TREND, CALIFORNIA

The gravity anomalies produced by the three Kettleman Hills Domes are maxima. The Lost Hills Anticline produces a gravity minimum. Four maps showing gravity contours, subsurface contours and sections accompany this paper. Maps from a gravity survey over the Kettleman Hills‐Lost Hills Area are compared with previously published subsurface contours and cross sections. These show that the three Kettleman Hills Domes each produce a strong and definite gravity high while the Lost Hills Anticline produces a very clear and closely corresponding gravity low. The paper points out the probable cause of this unusual feature as being a transition within the Reef Ridge Formation from punky, diatomaceous shale at Lost Hills to clays and sands at Kettleman Hills.

Gravity investigation of central-eastern Pennsylvania

In 1940 and 1941, 712 gravity stations were established in central-eastern Pennsylvania over a region centering about Allentown and Bethlehem. Geologically the area extends from the Triassic Lowlands to the Silurian of Kittatinny Mountain. Bouguer anomalies were computed, and from them gravity contour maps were made of the whole area and certain localities in it. Anomaly variation over the area is found to be both regional and local, the local anomalies correlating in shape with structural trends, and in several cases coinciding geographically with outcropping density contrasts that reasonably account for the gravity anomaly. Rough analysis is made of some of the structures, indicating the possible shape and size of mass anomalies that could produce the gravity anomalies found. Considerable overlapping of local effects makes detailed analysis difficult, but it is believed that the continued use of the gravimetric method in this region will be useful in studying its geologic and structural problems.

THE PLACE OF THE VERTICAL GRADIENT IN GRAVITATIONAL INTERPRETATIONS

It is a generally recognized fact that there is a certain similarity between structural contours and gravity contours. This is particularly true in the case of flat, shallow structures. In fact, in the limit, in the case of a surface distribution of matter, the gravity picture very near this surface becomes identical with the surface density distribution aside from a constant factor.