SubMAP Geophysical Services Sdn Bhd

28/01/2025

GOOD WELL-DISTRIBUTED DATA A CRITICAL FACTOR IN GEOPHYSICAL DATA PROCESSING/INTERPRETATION

A few days back, I saw a post showing a very nice smooth colorful contour map posted by someone. The first thing that struck my mine was how well was the data distributed since it was not shown ? I commented on the post requesting the author to show the data distribution. Unfortunately, the author provided a response which did not answer my question. Perhaps, I did not phrase the question correctly that he had issue in understanding the question.

As a geophysicist, I have this habit of not trusting a contour map until I've seen the data distribution. Even a single suspicious blip on the contour map, I would take the trouble to look into the issue to the extent of checking and verifying the field data.

I've observed numerous times and again, this common mistake of ignoring data distribution. Very common amongst newbies and those who are not trained in geophysics but are are engaged in carrying out geophysical surveys. Most of the time they trusted and accepted the outputs from the software without giving any thought to the data distribution. I have seen someone generating contour maps with only two survey lines located quite a distant from one another !

The image below shows another example from an ERT survey I personally completed recently. Earlier, my focus was only on the inverse resistivity section. Since I realized the importance of these pseudosections, I've made it a practice to include them when presenting ERT results. . It was obvious that there were bad data removed as observed on the measured and calculated apparent resistivity pseudosections. However, this was not reflected on the inverse model resistivity section. The question is : how much trust and confidence do you have on the inverse resistivity model, especially at the section where data were missing ?

Do not just simply trust and accept what the software is churning out. Pay special attention to data distribution; do not ignore them as they play a crucial role in deciding whether or not to trust the outputs.

It goes without saying that if you are into potential field modeling and inversions, you still need to have a very well distributed data set. f your data are sparse and not well distributed, the models would be very questionable.

25/01/2025

STAYING WITHIN THE LIMITS OF A NEAR-SURFACE GEOPHYSICIST WHEN SOLVING ENGINEERING ISSUES

Yesterday (24/01/25), I submitted a geophysical report to a client. Personally, it was really an unusual and interesting project.

The client is involved with a construction project. They had constructed a contagious bored pile (CBP) wall at the site. I have no idea what a CBP wall is and why it was constructed 😊. Somehow, the CBP wall was deflected ! This raised concern to the Client and their consultant. They were wondering what causes the deflection. Via their consultant, I was brought in to discuss whether electrical resistivity tomography (ERT) or any other geophysical methods were applicable to investigate the possible cause of the deflection. I was invited for a site visit. After the site visit, I had a meeting with both the Client and the consultant. It was decided to run an ERT survey and I briefed them on how it should be done. I'll also mentioned to them what would be the expected results.

It took about a month for the Client to finally made up their mind to run the ERT survey. However, the field data acquisition only took about half a day to be completed as there was only a single survey line with a 60m length.

The results of ERT survey showed promise revealing significant features that could explained the cause of the CBP wall being deflected. Here's the crunch though ! Being a pure bred near-surface geophysicist, how would I explained to the client the possible cause of the deflection without sounding like an engineer. As this is a geophysical report, I have to write it as a geophysicist sees it. At the same time, I would also have to provide sufficient input to the engineers so that they can use them to draw their own conclusion to the cause of the deflection on the CBP wall.

It was quite a tricky situation and I have to strike a balance between my point of view as a geophysicist and the engineers technical reasoning. Of course, the report should reflects a comprehensive geophysical interpretation. It should not also be too shallow for the engineers to come up with an engineering/technical explanation for the deflection.

I avoided encroaching into the engineers domain. That would spell disaster as I am not an engineer and it could also make me a laughing stock ! In the end I just focus on the properties of the subsurface geological materials which could be associated with the deflection of the CBP wall. Even that, it was purely based on a 'logical technical' thinking rather than a solid engineering justifications.

If it was a purely geophysical report, I would not face any issue. But trying to explain it in the point of view of an engineer, that would be too risky . I have to draw a boundary between my scope of work with that of an engineer.

Glad though that the report was accepted by the Client & their consultant without any revision to be done !

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15/12/2024

MEANINGFUL GEOPHYSICAL WORKS START WITH GOOD FIELD DATA !

It's pretty common nowadays to have near-surface geophysical (NSG) outputs in colorful 2D/3D images. In fact, I would say it's actually the norm rather than exception. To add to that, these are what clients love to see ! With the help of powerful software, generating these outputs are a breeze once the workings of the software is fully understood. The outputs can be generated in a fraction of a time which again pleases the clients.

In my many years of being a near-surface geophysical practitioner, it is extremely rare that clients would asked for the field (raw) data. Locally, this only happens when clients have their own geophysical consultant. Again, this is rare as typically locally clients do not engage geophysical consultant to act on their behalf. If they do engage consultant, it would often be either a geotechnical or geological consultant with little knowledge of geophysics. Personally, this is where the loop hole lies and where local geophysical contractors take full advantage to exploit.

It's critical to have good quality field data before proceeding to processing and interpretation. Clients are often presented with the final outputs. They have no idea of the data quality which has been acquired. What matters to them is the final output. Strangely, they often send their representative (so called Supervising Officer) to oversee the field data acquisition. However, they are just there to ensure that the field data specifications are followed; they are not trained to determine data quality.

A competent geophysicist or a well trained field operator is mandatory to handle geophysical field data acquisition. Even though clients are normally unaware and do not pay any attention to the importance of good quality data, it is the responsibility of the field geophysicists/operators to ensure that the data acquired are of 'good and acceptable quality'. The definition of 'good and acceptable quality' must be fully understood to avoid issues later in processing and interpretation. It's just not a matter of simply acquiring data without giving due considerations to the factors which affect field data quality.

Pay special attention to field data quality to avoid disastrous outputs !

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11/12/2024

PRODUCING COMPETENT NEAR-SURFACE GEOPHYSICISTS THROUGH A COMPREHENSIVE TRAINING/MENTORING PROGRAMS

Quite often by chance, I had the opportunity to review and provide second opinion on geophysical reports prepared by local contractors. These were normally provided by clients who wanted to know whether they have got their money worth carrying out geophysical surveys. The NSG works mainly focused on mineral & groundwater exploration and geotechnical/site investigations.

My conclusion is that : the need to undergo a comprehensive training in NSG has been seriously overlooked. There is an urgent need to make up for this by developing a comprehensive training program to produce competent geophysicists in future.

The million dollar question is : whose role/responsibility is this ?

Newbies and fresh graduates simply do not have the required knowledge & skills to quickly be turn into competent geophysicists overnight. When they are first employed, they need to be properly guided and carefully nurtured by someone with good knowledge & experience in NSG. It is essential that they go through a well developed training programs. However, this is often not the case. They are usually train in-house by seniors where they are attached to via on the job training. However, there is a real risk in this mode of training. If they have seniors supervising them who are not competent in NSG, the knowledge passed down to them will be little use to mould them into competent geophysicists. In a worst case scenario, he/she will pass down this knowledge further down the line to the younger generation.

I have often being approached by students pursuing advance degrees in NSG and also practitioners with advance degrees , requesting for advice & assistance, especially in data processing. From my countless online conversations with them, it's pretty obvious that the lack of understandings of the basic fundamentals of the specific geophysical methods is glaring ! Personally, even with advance degrees, they still need to undergo training to join the industry. Having advance degrees is not a ticket to being a competent geophysicist; it's a good start though.

Locally, if this trend continues, it will just dampen the NSG industry. Clients who have been short changed with shoddy geophysical works will shy away from engaging in geophysics in future. They will be a huge trust deficit in geophysics. The signs are already there. What keeps NSG going in the local industry is that most clients have no/little knowledge of geophysics. If you can provide them with colorful 2D/3D images and talk your way out, they will be definitely be impressed; until a competent geophysicist pointed out the technical wrong doings. Only then they will realized that they have been taken for a ride and their investment gone to waste. By then, it's often, already too late.

Let us turn NSG into a locally healthy competitive industry. Ensure that clients have their money worth when engaging geophysics !

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30/11/2024

LAYERING IN 3D GRAVITY INVERSION

3D gravity inversion showing subsurface layering.

The 3D images below are from the same data presented in two different voxel formats. The left image show the layers in 3D voxel isosurface whereas the image on the right is presented in 3D voxel thick slices. The isosurface represent layers of equal densities. In addition, the drop in bedrock's depth is also clearly visible. This has been proven with borehole data.

These 3D inversions are carried out with HORIN's 3D potential field inversion.

If you are interested to try the software, the trial/evaluation version of the software in available for download via the following link :
[https://lnkd.in/g2DHa9X3](https://lnkd.in/g2DHa9X3)

19/11/2024

18/11/2024

SEISMIC REFRACTION SURVEY ON A DISPUTED ISLAND

Years ago I was involved in a seismic refraction survey on a disputed coral island, located far off in the middle of no where. This small coral island, slightly larger than the size of a football field, was claimed by several countries. The journey to the island took us more than 6 hours by ship.

This in fact was my first seismic refraction survey deploying hydrophones. The two lower images below, show the laying out of the hydrophones and cables. The survey was conducted on shallow waters close to the perimeter of the island using small amount of explosives as the energy source. The effect of the blast can be seen on the two upper images.

The seismic refraction survey was carried out for the proposed construction of a runway for planes. No idea whether this project was implemented though.

How I miss the days of being a field geophysicist !

16/11/2024

WHEN SHOULD YOU CARRY OUT 3D INVERSION OF POTENTIAL FIELD DATA ?

Several of my recent posts have been focusing on 3D inversion of potential field data (both gravity and magnetic) using HORIN's geophysical software. Following these posts, I have received several requests to evaluate the software by downloading the trial (evaluation) version. Anyone interested to try the software can download it via the following link :

https://lnkd.in/eqwdDXPp

When you have downloaded & install the software, you will be prompted for an activation code to run it. Please provide me with the Hardware ID, I will then send you the activation code.

Back to my post, when should you run 3D inversion on potential field data ?

I was recently approached by a connection requesting to try out Horin's 3D potential field inversion. She was doing a gravity project to map mud volcanoes. In her eagerness, she went straight into doing the 3D inversion as I believed she was very excited seeing what I've done. When she got stuck with the 3D inversion, she requested my assistance. That was when I started probing what she has done.

Apparently, she did not even bother to focus on the bouguer/anomaly maps and do some interpretation before proceeding with the 3D inversion. It was plain obvious that she was totally off-track. She mentioned that she got the data from her lecturer. I just couldn't believe my ears that she was not fully guided by the lecturer.

Before, even considering to carry out 3D inversion, there are few pointers which should not be ignored. The first is to ensure that you have good data distribution. Your data should cover the entire site without leaving too many gaps and blanks without data. If not, there will be too much interpolation by the 3D inversion software.

It is critical to carry out interpretation on your primary & secondary maps (after advance processing) to construct a subsurface model. You should develop an acceptable subsurface model based on the features observed by extracting as much information as possible from both the primary & secondary maps. You can even run a simple 2D modeling to have an idea of the depth & dimensions (shape and size) of your target. Only then you should think whether to run 3D inversion.

It does not make a lot of sense to run a 3D inversion exercise, when you have not develop a decent subsurface model. Once you understand and has a certain degree of confidence in your subsurface model, only then proceed to do 3D inversion

Please do not carry out 3D inversion for the sake of pleasing clients. Clients are always impressed with 3D outputs. However, as professional geophysicists, you should only carry out 3D inversions if the models provide additional & useful information beneficial to the clients, not to just impress them !

This link will take you to a page that’s not on LinkedIn

03/11/2024

ROADMAP OF A NEAR-SURFACE GEOPHYSICIST

(i) FIRST 5 YEARS : Learning stage. Observe and learn !

(ii) NEXT 5 YEARS : Carry out as many geophysical projects as possible in different environments and terranes.

(iii) NEXT 5 YEARS : Lead and manage geophysical projects. By this time, you should be competent enough to be given the responsibility to handle any geophysical project.

(iv) NEXT 5 YEARS : Be a mentor as it's payback time ! Impart knowledge & experience to the younger generations.

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01/11/2024

3D GRAVITY INVERSION OF A STEEP SIDED LIMESTONE BEDROCK USING HORIN GEOPHYSICAL SOFTWARE

The images below are from a microgravity survey which I did for a Client many years ago. I was not able to do any inversion those days. With the availability of HORIN Geophysical Software, I carried out a 3D inversion using the same data, just to see the output.

One nice feature of this software is the comparison of the fit between the raw and inverted data (left images) in 2D. The residual is also displayed. This will certainly assist in deciding whether to accept the 3D inverted model (right). The goodness of fit between the two images will lend confidence in the reliability of the 3D model which shows steep-sided limestone bedrock.

Interested ? Drop me a message !

28/10/2024

THE NEED OF BOREHOLE CONTROLS IN GRAVITY ROCKHEAD MAPPING

The images below show the 2D rockhead maps at the same site. The image on the right was generated based on three (3) borehole logs available at the time of doing the rockhead modeling almost 19 months ago. Whereas the image on the left is a recent 2D rockhead map which was revisited with additional four (4) borehole logs made available by the Client.

Yup, of course there are similarities and also stark differences between the two images. Just to prove a point that the issue of non-uniqueness (ambiguities) in gravity (& also magnetic) is not to be overlooked. As more boreholes are drilled, the rockhead models can be refined bringing them closer to the actual subsurface.

Despite the issue of ambiguities, gravity (microgravity) is still a viable tool to map rockheads, especially in urban built-up areas. With adequate borehole controls, it can provide valuable data/info of the subsurface.

24/10/2024

WHERE IS THE LIMESTONE/GRANITE CONTACT ?

Among the many applications of gravity (microgravity) is to determine the contact/boundary of rock types.

Many years ago, I was involved in a microgravity survey to demarcate the contact between limestone and granite. It was a very interesting case study as limestone was not known to exist at the site. The whole site was thought to be underlain by granite; not until piling works commenced. When piles driven when missing, only then the consultants realized that they were dealing with cavities in limestone !

The result of the microgravity survey is shown in the image below. Can you figure out where the is the boundary between limestone and granite 😉 ?