It's simply related with the difference between seismic method and gravity method in vertical and lateral resolution. Generally speaking, seismic method has a much higher resolution than the gravity method. However, the gravity method is much cheaper and is much easier to detect those gravity anomalies in a large area. I would suggest using gravity method to get a general understanding of the gravity properties of a large area, and their related structural assumptions. Then, using seismic method to probe the vertical and lateral extension of a fault in a local area. It is also the common way to combine these two methods in the geophysics community.  

hi

the resolution (precision) of the measured gravity and altitude data are very important. Also, the gravity data corrections must make correctly,  particular  terrain (topography) correction. It is important that be noted, the density contrast of the two side of fault  must be enough to be detected by gravity data.  

The gravity data can be used to delineate faults, see the reference

Khattach D., Houari M. R., Corchete V., Chourak M., El Gout R. and H. Ghazala, 2013. Main crustal discontinuities of Morocco derived from gravity data. Journal of Geodynamics, 68, 37-48.

The seismic resolution compared with gravity is relatively high moreover, the seismic techniques are sensitive to both lateral and vertical changes in the physical elastic properties of soil and rock and the velocity of mechanical waves in soil or rock depending on many factors e.g. type of rocks/soil as well as the situation fractured weathered the orientation of the joints, fractures , the size if the voids and matrix materials  as well as existing fault, type of fault, fault throw, shear zone and finally, the thickness and the nature of filling materials which can play a big role in the capability of the methods to detect the mentioned structure.   

it is recommended to carry out one line by using GPR over the mentioned lines by applying mutli frequency antenna.  

Seismic will definitely provide a higher resolution image that helps to delineate faults in the subsurface. But it is also a more expensive technique, in terms of money and logistics. Gravity surveys can also be used, if the faults have significant amount of throw and adjacent rocks show detectable differences in density.

In fact, the classical work on rift basins delineating horst and graben were done only using gravity datasets, predating the advent of seismics. Early oil and gas discoveries in 20th century were also made with gravity only, and faults throws were routinely detected.

In principle, both techniques should compliment. Look at the attached paper, where while gravity and seismic compliment, but not perfectly in figuring out the subsurface.

Generally, gravity is a good tool in fault detection in the case that you have a significant value of density contrast on the grounds of all other conditions are ok. Anyway seismic is the best.

Gravity method can detect faults which enough large effect on gravity data , sử can use gravity data to detect boundary geology.

I think it is possible to detect subsurface faults specially having great throw of significant density contrast. But we could detect subsurface faults from seismic sections even in case of no clear fault significance. 

i agree with Hosni Ghazala, the method gravity only show boundary geology which difference contrast density

I think generally, gravity is a good tool in fault detection in the case that you have a significant value of density contrast on the grounds of all other conditions are ok. Anyway seismic is the best.

Your main problem that the used theory of gravity is wrong!… That is why you cannot use it totally successful. Exist possibility to develop a better method… Which can be used well with 2 D- 3 D seismic diagrams and can give an excellent opportunity to find hidden fault. Only have to be more interested for such a “ideas”… I have two articles in Hungarian written them in Chinese cloth magazine (can be download here in RG)… From them you can get some suggestion perhaps… Practical utility are not written, perhaps in condition more respectable (example for a MsC. geologist engineer) they can be disputed  more efficiently.

Best Regards,

Laszlo,  

The main issue in geophysics is "contrast". As far as  utilizing gravity and seismic refraction in detecting faults is concerned, one considers the fault type, depth, size and density contrast. If the contrast is present, both methods will work with an accuracy that depends on the mentioned variables, mainly the density contrast. If this contrast does not exist both methods will not work. This might be the case for some strike-slip faults.

Hi, dear professor Aboud , The accurate spacing for gravity survey is very effective for detect the subsurface sources. I think two or three meters spacing for gravity survey is suitable to detect the sources at depth range 6-8 meters. Spacing is only one parameter; you know the influence of accuracy of measurements, drift, accuracy of elevation, corrections, separations of residual anomaly, and contour interval of residual map. Gravity method can give good results with the accuracy in all steps from the field survey to the final interpretation stages.

Klaus Helbig

gravity observed is the sum of the effect by ALL masses in the universe (most of them, fortunately, neglgible because of distance), thus at least a volume integral over the surroundings. In contrast, seismic observatirons will show theffect of displacements greater than about a quarter of the effective wavelngth.

Klaus Helbig is right.  I have nothing more to add.

You need to thing in the physical difference(density and acustic impedance) of the method, that can only with contrast can work; and the study of the geology of the area is very important to the interpretation.

Aboud, what you raise is really uncommon. In its theoretical development, gravimetry would allow detecting vertical contacts with relative ease, of course, if there is an adequate density contrast on both sides of it. On the contrary, seismic, offers its best results when we try to explain a horizontal sequence of strata in depth, therefore and although in fact it does, it would not be its strong detection of vertical contacts. In your case, the spacing used seems to be very detailed and for that reason, if the vertical contact is close to the surface should not go unnoticed for gravimetry.

To answer this question, you must go back to the factors affecting each method. Gravity method based on depth and density which put the gravity method under ambiguity phase and then, the structures determined by gravity method is not completely right due to the effect of density with that of depth. But with seismic method these phenomena are absent and then we can conclude that, the structures determined by seismic method is more accurate than that determined by gravity methods.

Gravity methods based on depth and density. From the ambiguity of gravity method is the confusion to know the variation of the gravity method is due to depth or density. Finally, gravity method can detect faults but not in precision.